USPS flat rate boxes let you ship anything up to 70 pounds for a single fixed price, regardless of where it’s going in the United States. The cost depends only on the box size you choose, not the weight of the package or the distance it travels. A 50-pound box of books going from coast to coast costs the same as a 3-pound box going one state over, as long as both fit in the same size box.
How the Pricing Works
Flat rate shipping is part of the USPS Priority Mail service. You pick a box size, pack your items inside, and pay a set price. That’s it. There’s no need to weigh the package or calculate shipping zones. The retail prices as of January 2026 are:
- Small Flat Rate Box: $12.65
- Medium Flat Rate Box: $22.95
- Large Flat Rate Box: $31.50
If you ship through a commercial platform like an online store or a shipping service that offers commercial rates, the prices drop to $11.20, $19.60, and $28.70 respectively. You get the same Priority Mail delivery speed either way, typically one to three business days.
Box Sizes and What Fits
USPS offers several flat rate box options, each designed for different types of shipments. The small box has interior dimensions of roughly 8 5/8 by 5 3/8 by 1 5/8 inches, making it good for small, dense items like jewelry, coins, or phone accessories. Think of it as slightly larger than a paperback book.
The medium box comes in two versions. The top-loading version (about 11 by 8 1/2 by 5 1/2 inches inside) works well for stacking items. The side-loading version (about 13 5/8 by 11 7/8 by 3 3/8 inches inside) is flatter and wider, better for documents, clothing, or anything that lies flat. Both cost the same.
The large flat rate box measures about 12 by 11 3/4 by 5 1/2 inches inside. There’s also a large flat rate board game box with a long, shallow shape (roughly 23 11/16 by 11 3/4 by 3 inches inside), useful for board games, framed items, or anything with that kind of footprint. An APO/FPO/DPO version exists for military addresses, with the same dimensions as the large box.
Getting the Boxes
The boxes themselves are completely free. You can order them from the USPS website and have them delivered to your door at no charge, or pick them up at your local post office. They come printed with “Priority Mail Flat Rate” branding, which is what qualifies them for flat rate pricing. You cannot use a random box from your garage and pay the flat rate price. It has to be the official USPS flat rate box.
Rules for Packing
The main rule is simple: if it fits, it ships. But the box has to close properly along its original folds. You can tape the flaps and reinforce the seams, but you cannot cut, reshape, or reconstruct the box to make it bigger. If you open the sides and re-tape them to squeeze in an oversized item, the post office can reject the package or charge you a different rate.
There’s no restriction on what you put inside (beyond standard USPS prohibited items like hazardous materials). Heavy, dense items like tools, canned goods, or machine parts are perfectly fine as long as the box stays under 70 pounds and closes on its normal fold lines.
When Flat Rate Saves You Money
Flat rate boxes are most valuable when you’re shipping heavy items over long distances. Standard Priority Mail charges are calculated by weight and shipping zone, so a 30-pound package going across the country can get expensive fast. With flat rate, that same package costs the fixed price no matter what.
The savings flip, though, for lighter packages traveling shorter distances. If you’re shipping something under 20 pounds and it’s only going a few zones away, weight-based Priority Mail in your own packaging will often be cheaper than the medium or large flat rate box. For the small flat rate box, weight-based rates only beat it for very light packages traveling very short distances.
A quick way to decide: if your item is heavy relative to the box size and your recipient is far away, flat rate almost certainly wins. If it’s light and the destination is nearby, compare it against standard Priority Mail rates before committing. USPS offers a price calculator on its website where you can enter the weight, dimensions, and destination ZIP code to see the weight-based alternative.
How to Ship a Flat Rate Box
Start by ordering or picking up the right size box. Pack your items securely with cushioning material to prevent movement. Seal the box with packing tape along all seams. You can then pay for postage and print a label at usps.com, through a shipping platform, or at the post office counter. Drop the box at any post office, hand it to your mail carrier, or schedule a free package pickup from your address.
Every flat rate box includes tracking and up to $100 in insurance at no extra cost. If you need more coverage, you can purchase additional insurance when you buy the postage.