USPS Flat Rate Boxes, Envelopes, and Prices

USPS Flat Rate boxes and envelopes let you ship up to 70 lbs via USPS Priority Mail® for one price to anywhere in the US. The appeal is simple: if it fits and the box closes normally, it ships for the same rate, no matter the weight or destination. Flat Rate can be one of the cheapest ways to ship, but standard Priority Mail, USPS Ground Advantage®, or other services may cost less for lighter items.

Knowing when to use Flat Rate—and when not to—can save you time and money on every shipment.

What Is USPS Flat Rate?

USPS Flat Rate is a pricing model under Priority Mail® in which you pay a single flat fee to ship packages or envelopes of specific dimensions, regardless of weight (up to 70 lbs) or destination within the U.S.

These USPS Priority Mail Flat Rate services include:

• Flat Rate Envelopes (standard, window, small, legal, and padded)
• Flat Rate Boxes (small, medium, and large)
• APO/FPO/DPO options for military and diplomatic addresses

Because rates are fixed, USPS Flat Rate shipping is ideal for heavy shipments moving long distances. The rule to remember is “if it fits, it ships”: as long as the flaps close within the normal folds, you pay the Flat Rate price. Flat-rate packaging is also exempt from USPS nonstandard fees.

Where to get USPS Flat Rate boxes

USPS Flat Rate boxes and envelopes are free. You can order them online from the USPS store or pick them up at most Post Office locations. Just make sure you use the packaging that corresponds to the Priority Mail Flat Rate service. Misusing flat-rate packaging, such as sending it via USPS Ground Advantage®, Media Mail®, or standard Priority Mail, can result in returned parcels or billing corrections.

When you can’t use USPS Priority Mail Flat Rate shipping boxes

USPS Flat Rate packaging cannot be used with standard Priority Mail services, USPS Ground Advantage®, First-Class Mail®, Media Mail®, or other carriers. They are only allowed for their designated Priority Mail Flat Rate service. If you try to use a Flat Rate box or envelope with a standard Priority service or any other service, the parcel will be returned, or a post-billing correction will be sent to your postage account.

USPS Flat Rate box prices

Priority Flat Rate is the USPS’s one-size-fits-all packaging that lets you fit as much as you can into the box as long as it’s up to 70 lbs. The prices for Priority Mail Flat Rate and the dimensions of each package type are listed below.

Prices below are current as of October 2026 and include USPS’s temporary price adjustment, which runs through January 17, 2027. USPS sets the final rates, so confirm at usps.com before you ship.

USPS Flat Rate box sizes and envelope options

Here’s a breakdown of USPS Flat Rate box sizes, flat rate envelope types, and current pricing:

* Outer dimensions shown. Inside dimensions are slightly smaller, so measure bulky items before you pack.** Retail rates are those available at the Post Office. ShipStation provides discounts on select USPS services.

Cost of Priority Mail Express Flat Rate envelopes

Currently, USPS offers a few flat-rate options for its Priority Mail Express service. This is a faster alternative to Priority Mail Flat Rate, with 1–3 day delivery and a money-back guarantee (see our guide to overnight shipping options).

With a maximum weight of 70 lbs, you can still use the USPS Flat Rate envelope, Legal Flat Rate envelope, or Padded Flat Rate envelope. However, like Regional Rate boxes, the USPS has discontinued its Priority Mail Express Flat Rate Box offerings.

USPS Priority Mail Express Flat Rate envelopes

For urgent shipments, USPS offers Priority Mail Express Flat Rate envelopes. These provide 1–3-day delivery, a 70-lb weight limit, tracking, and up to $100 insurance on most shipments.

Flat Rate vs. Standard Priority Mail: When to choose what

Standard USPS Priority Mail works for parcels up to 70 lbs that don’t exceed 108″ in combined length and girth (girth is the distance around the thickest part). These parcels arrive in the same 2–3 day timeframe as Flat Rate options, but the pricing is different.

Flat-Rate pricing ignores weight, dimensions, and delivery zone. Standard Priority Mail pricing factors in all three, and packages over 1 cubic foot may be charged by dimensional weight. For lighter parcels, it’s usually less expensive to use your own packaging and get a rate. For small, dense packages, Priority Mail Cubic can beat Flat Rate, and for very light packages, USPS Ground Advantage® is often the better deal.

For example, a 3-lb package from NY to CA will likely cost less shipped in your own box than in a USPS Medium Flat Rate Box, which is why it pays to compare rates before you print.

USPS Flat Rate international shipping

USPS also offers Flat Rate International options through Priority Mail International and Priority Mail Express International. These let you send packages to about 180 countries.

Weight limits are lower than domestic: 4 lbs for envelopes and Small boxes, and 20 lbs for Medium and Large boxes. Tracking and insurance (up to $200 for merchandise) are included with some exceptions, including no tracking on Flat Rate envelopes or Small boxes, and limits vary by country.

Tip: Use ShipStation’s multi-carrier tools to automate customs documentation and compare international flat rate options from USPS and other carriers.

Save on USPS Flat Rate shipping with ShipStation

Using flat rate shipping boxes is already cost-effective—but ShipStation helps you save even more. Through ShipStation Carrier Services, you get access to: 

• Up to 89% off USPS retail rates.
• Pre-built shipping automation rules.
• Multi-carrier rate comparisons.
• Batch label printing & scheduled pickups.

Whether you’re using USPS prepaid boxes, padded mailers, or shipping by weight, ShipStation makes it easy to ship smarter, not harder.

James Messer

James Messer is a copywriter specializing in shipping, logistics, and ecommerce.

More articles by James Messer