Canada Post Flat Rate Box Sizes & Prepaid Boxes: 2026 Guide

For small sellers, the biggest headache is shipping costs that change with every parcel’s weight and destination. Canada Post has tried to quell these issues through prepaid packaging, meaning Canada Post flat rate boxes, prepaid envelopes and prepaid labels.

In this guide, the focus will be on discussing the box sizes, the package prices in 2026, and when a flat rate can help sellers save money.

What Are Canada Post Flat Rate and Prepaid Boxes?

What is a Canada Post flat rate box? In simple words, a Canada Post flat rate box is a ready-to-use box with postage built into the price. Postage isn’t based on your parcel’s destination or weight, up to 5 kg.

One thing to keep in mind is that when people talk about prepaid and flat rate boxes, they are talking about the same thing.

A flat rate box comes in four sizes, ranging from extra small to large, but all are capped at 5 kg (11 lb). Single boxes can be bought from any post office. Online, however, buyers can only get them in packs of 12, as there is no way to buy a single box on the internet.

The price includes postage, the box itself, tracking, an on-time guarantee, up to $100 in liability coverage, and carbon-neutral shipping.

How are they different from other prepaid products? There are two other options:

• First are prepaid envelopes. They are pre-priced Priority/Xpresspost mailers for documents and light, flat items.
• Second are prepaid labels. They are pre-bought postage that you stick on your own box. They are meant for Priority, Xpresspost, and Expedited Parcel.

Small eCommerce sellers don’t need a scale at drop-off. They pay the same cost for every order within the weight limit. That makes this option suitable for gifts, subscription boxes and flat-fee product lines.

Canada Post Flat Rate Box Sizes

Canada Post flat rate box sizes come in four standard options. All four share the same 5 kg cap, and each price already includes postage to anywhere in Canada.

Size	Dimensions (L × W × H)	Max weight	Price (CAD)	Drop-off	Typical use case
Extra Small	22.5 × 15.5 × 7.62 cm (8⅞ × 6⅛ × 3 in)	5 kg	$18.99	Street letter box or post office	Jewellery, cosmetics, phone accessories
Small (2026)	32 × 24 × 8 cm (12⅗ × 9⁹⁄₂₀ × 3³⁄₂₀ in)	5 kg	$21.99	Street letter box or post office	Books, folded apparel, small electronics
Medium	37.9 × 26 × 12 cm (14¹⁵⁄₁₆ × 10¼ × 4¾ in)	5 kg	$24.99	Post office	Multi-item orders, shoes, gift sets
Large	40.3 × 29.8 × 18.7 cm (15⅞ × 11¾ × 7⅜ in)	5 kg	$32.99	Post office	Subscription boxes, bulky bundles, dense goods

So which one should you pick? Here is how these Canada Post box sizes and prices work out in practice.

The extra small box and the small flat rate box Canada Post redesigned for 2026 both fit into a street letter box, so sellers can skip the trip to the counter. The large flat rate box Canada Post offers is a different story. Not every location stocks it, but you can ask for one at your post office.

The weight limit doesn’t change with size. Every box tops out at 5 kg (11 lb), and Canada Post won’t ship one that goes over. If your item is heavier, you will need a weight-based service like Expedited Parcel, Xpresspost or Priority, where the cost depends on weight and destination.

One thing to keep in mind is that the box has to stay as it is. You can’t cut it down, extend it or change its shape, so whatever you ship has to fit the box as it comes.

When choosing between these Canada Post standard box sizes, go by the bulkiest item you ship regularly rather than the average one. A box that’s too small means repacking at the counter. A box that’s too big means paying the Canada Post flat rate box price for space you don’t use.

Finally, Canada Post revises its packaging and pricing from time to time, and the redesigned 2026 Small box is an example. Before ordering in bulk, check the current dimensions and prices on the Canada Post flat rate boxes page or at your local post office.

Canada Post Prepaid Envelopes and Labels Explained

Flat rate boxes are not Canada Post’s only prepaid option. When an item doesn’t suit a box, there are prepaid envelopes and labels. Together, Canada Post prepaid shipping boxes and envelopes cover most of what a small seller sends.

Prepaid Envelopes

Prepaid envelopes work with Priority (next-day delivery) and Xpresspost (next day locally or regionally, two days nationally). They come in regional versions, for shipping within the same region, and Canada-wide versions.

Commercial envelopes arrive with the label and tracking barcode already attached, so you only have to fill in the details and drop them off. Xpresspost sizes run from 260 × 159 mm (500 g) up to 394 × 314 mm (1 kg). They suit contracts, documents and light, flat items.

Prepaid Labels

Prepaid labels are prepaid postage for Priority, Xpresspost or Expedited Parcel that you stick on your own packaging. They suit brands that use custom boxes but still want a predictable cost. The catch is that you need a Standing Offer Agreement to get them. You also have to order them ahead of time, since Canada Post prints your name and address on each label. They can’t be used for oversize or unpackaged items, or for deliveries to Nunavut, the Northwest Territories or Yukon.

One thing to keep in mind is that Canada Post prepaid box sizes refer to the four flat rate boxes above. Envelopes and labels have their own specs, listed in the Parcel Services Customer Guide.

There’s also a time limit. Canada Post prepaid boxes, envelopes and labels should be used within three years of purchase, after which delivery isn’t guaranteed. Bulk buyers should plan around this.

Where and How to Buy Canada Post Flat Rate and Prepaid Boxes

So, where to buy Canada Post flat rate boxes? You have two options. Single boxes are sold at any post office counter, though you may need to request the large size. Online, boxes come only in packs of 12, delivered to your door, which suits sellers stocking up before a busy season.

Don’t expect a discount for buying more. The price per box stays the same whether you buy one or twelve, even with a Solutions for Small Business or commercial account. Signature and extra coverage add-ons aren’t available either.

Prepaid envelopes are sold at post offices or at canadapost.ca/shop. Prepaid labels go through Canada Post customer service or a sales rep, and they need a Standing Offer Agreement plus some lead time for printing. Canada Post’s business prices guide covers commercial pricing in more detail.

A tip for eCommerce sellers: the prepaid boxes Canada Post sells work best when you stick to one or two sizes. Packing gets quicker, and you can show one shipping fee at checkout.

Flat Rate Boxes vs. Weight-Based Shipping: Which Should You Use?

When it comes to Canada Post flat rate vs weight-based shipping, the choice comes down to one thing. With flat rate shipping, you are paying for certainty. Whether that’s good value depends on how heavy your item is for its size and how far it’s going.

Criteria	Flat Rate Box	Weight-Based Shipping
Pricing basis	Fixed per box	Weight, dimensions, destination
Max weight	5 kg	Up to 30 kg (by service)
Packaging	Canada Post box only	Your own box
Coverage	Canada only	Canada, US, international
Best for	Dense, heavy items, long distances	Light items, regional, oversize
Add-ons (signature, extra coverage)	Not available	Available
Scale needed	No	Yes

Flat rate tends to win with dense or heavy items close to the 5 kg limit, especially when they’re travelling a long way, such as from Ontario to BC. For a box like that, weight-based Xpresspost or Expedited Parcel would usually cost more.

Weight-based shipping wins with light, small items and short regional trips. If the item fits a prepaid envelope or a small custom mailer, a Canada Post flat rate box will likely cost more than it needs to.

For small sellers, there’s another factor. Expedited Parcel is only open to customers with a Standing Offer Agreement or a Solutions for Small Business card. If you ship occasionally without an account, flat rate may be more competitive than it looks on paper.

Here’s a simple rule. If your item is heavy for its size and fits one of the four boxes, flat rate is usually the better deal. If it’s light or oddly shaped, get a weight-based quote first.

Also remember that flat rate only works within Canada. Shipments to the US or abroad use weight and zone-based pricing through services like Xpresspost – USA or International Parcel.

Frequently Asked Questions

How much is a flat rate box Canada Post?

A Canada Post flat rate box costs between $18.99 and $32.99 in 2026, depending on the size. The Extra Small box is $18.99, the Small is $21.99, the Medium is $24.99 and the Large is $32.99. That price covers the box and postage to any address in Canada, as long as the box weighs no more than 5 kg. So whether you send a parcel across town or across the country, the cost stays the same. Don’t expect a lower rate for buying in volume. Even Solutions for Small Business and commercial account holders pay the same price per box. Since Canada Post revises its prices from time to time, check the current rates at canadapost.ca or your post office before placing a bulk order.

Can you buy Canada Post flat rate boxes online?

Yes, but not one at a time. Canada Post only sells flat rate boxes online in packs of 12, which are shipped to your address. That works well for sellers who want to stock up before a busy season. If you need just a single box, you will have to buy it at a post office counter. Keep in mind that the Large box isn’t stocked at every location, so you may need to request it. Other prepaid products work differently. Prepaid envelopes can be ordered online at canadapost.ca/shop, while prepaid labels go through Canada Post customer service or a sales rep. Labels also need a Standing Offer Agreement and some lead time, since Canada Post prints them with your name and address in advance.

What is the maximum weight for a Canada Post flat rate box?

Every Canada Post flat rate box has the same maximum weight of 5 kg (11 lb), whether it’s the Extra Small or the Large. The size of the box changes, but the weight cap doesn’t. Canada Post won’t ship a flat rate box that goes over the limit, so it’s worth weighing heavier orders before you drop them off. This is especially true for the Medium and Large boxes, which are easy to overfill with dense items like books or tools. If your item weighs more than 5 kg, you will need a weight-based service such as Expedited Parcel, Xpresspost or Priority instead. With these, the price depends on the parcel’s weight, size and destination rather than a fixed rate, so get a quote first.

Do Canada Post flat rate boxes come with tracking?

Yes, they do. Every flat rate box comes with tracking, so you and your customer can follow the parcel right until it’s delivered. It also includes an on-time delivery guarantee, although the delivery standard varies by destination and starts when the box is first scanned. On top of that, you get up to $100 in liability coverage for loss or damage, subject to Canada Post’s conditions. What you can’t do is add extras. Signature options and additional coverage aren’t available on flat rate boxes, so high-value items may be better off with another service. For most everyday orders, though, the built-in tracking and coverage are enough. Prepaid envelopes work the same way, as they also include tracking and delivery confirmation along with the on-time guarantee.

Can I use a Canada Post flat rate box to ship to the US?

No. Flat rate boxes are meant for addresses within Canada only, so you can’t use one to send a parcel across the border. For US-bound shipments, Canada Post uses weight- and zone-based pricing through services like Xpresspost – USA. International orders work the same way through services like International Parcel. In both cases, the price depends on the parcel’s weight, dimensions and destination, so it varies from one order to the next. You also need to use your own packaging rather than a flat rate box. If you sell to customers on both sides of the border, it helps to keep flat rate boxes for domestic orders and quote US and international orders separately at checkout. That way, your Canadian customers get a predictable shipping fee.

How long are Canada Post prepaid boxes valid?

Canada Post recommends using its prepaid products within three years of buying them. After that, it can’t guarantee delivery. The rule covers flat rate boxes, prepaid envelopes and prepaid labels alike. This matters most for sellers who buy in bulk. A stack of 12-packs might seem like a good way to prepare for busy seasons, but stock that sits around too long can become a problem. Your product line might also change in that time, leaving you with boxes that no longer fit what you sell. The safer approach is to buy based on how many boxes you actually ship in a few months, and to use your older stock first. A simple first-in, first-out system on your packing shelf makes this easy to manage.

What is the difference between a prepaid box and a prepaid label?

A prepaid box, also called a flat rate box, is a Canada Post box with postage already included in the price. It comes in four fixed sizes and can carry up to 5 kg anywhere in Canada. A prepaid label is pre-bought postage for Priority, Xpresspost or Expedited Parcel that you stick on your own packaging. Labels need a Standing Offer Agreement, are printed with your name and address, and can’t be used for deliveries to Nunavut, the Northwest Territories or Yukon. They also need to be ordered in advance. In short, sellers with branded boxes are better off with labels, while those who want simple, predictable shipping without a contract should go with boxes. Many growing businesses end up using a mix of both.

Conclusion

Canada Post flat rate boxes make shipping a lot simpler. There are four sizes, every box is capped at 5 kg, and the price already includes postage to anywhere in Canada. The only limit is that they don’t work for parcels headed to the US or overseas.

Whether they save you money depends on what you sell. Flat rate works best for dense, heavy items travelling long distances within Canada. For light parcels, it’s worth comparing weight-based rates before you commit to a box.

That’s where ShippingChimp can help. Instead of checking every option by hand, you can compare Canada Post rates in one place and pick the most cost-effective way to ship each order.