A 40 pound box and a 4 pound box cost exactly the same under Priority Mail flat rate pricing. That holds as long as both fit in the same box, since weight simply stops mattering once you commit to a flat rate size.
That single fact makes flat rate a great deal for some shipments and a bad one for others. The trick is knowing which category your specific package falls into.
Here is every Priority Mail flat rate option, current pricing, and exactly when flat rate beats weight based pricing.
Every Priority Mail flat rate option and price
| Flat rate option | Retail price | Interior dimensions |
|---|---|---|
| Flat Rate Envelope | $12.90 | 12.5 x 9.5 in |
| Legal Flat Rate Envelope | $13.25 | 15 x 9.5 in |
| Padded Flat Rate Envelope | $14.00 | 12.5 x 9.5 in, padded |
| Small Flat Rate Box | $13.65 | 8.625 x 5.375 x 1.625 in |
| Medium Flat Rate Box | $24.80 | 11.25 x 8.75 x 6 in |
| Large Flat Rate Box | $34.00 | 12.25 x 12.25 x 6 in |
Every flat rate box and envelope ships up to 70 pounds at that same fixed price. Destination within the country never changes it. That is the entire appeal: one price no matter how far the package travels.
How flat rate pricing actually works
Regular Priority Mail charges by weight and distance, using USPS zone charts. Flat rate skips both of those variables entirely and charges based only on the box or envelope you choose.
The only real requirement is that the item fits inside the box and the box closes properly without bulging. A box taped shut over an overflowing lid does not qualify for flat rate pricing.
When flat rate saves money
Flat rate wins for heavy, dense items headed a long distance. Weight based pricing would otherwise charge more as both weight and zone climb. A box of books shipped cross country is a classic example.
It also wins for anything close to the weight limit. A 40 or 50 pound item would cost far more under standard zone based pricing. Our full guide to Priority Mail covers how zone based pricing works for comparison.
When flat rate costs more than necessary
Light items shipped a short distance usually cost less under regular weight based Priority Mail pricing. A 1 pound item going a few states over does not need a $13.65 flat rate box. Weight based pricing might run less for that exact shipment.
Small, low weight shipments are also often cheaper under USPS Ground Advantage entirely, skipping Priority Mail altogether. Our guide to Ground Advantage covers that lower cost option. Our stamp calculator and postage tools can help compare exact costs before you choose.
Where to get flat rate boxes and envelopes
Every flat rate box and envelope is free at any post office. No purchase is required to take them home. USPS.com also lets you order supplies for home delivery at no charge.
Stock up ahead of a shipping heavy period, like a small business handling holiday orders. The boxes themselves cost nothing regardless of how many you take.
Flat rate versus regional rate boxes
Regional rate boxes are a separate, lesser known option. They price by weight and a broader distance zone rather than a single flat price nationwide, and they can beat standard flat rate pricing for shorter distance shipments specifically.
Most individual shippers never encounter regional rate boxes, since they are mainly used by high volume commercial accounts. Flat rate remains the simpler, more widely available option for most people.
Frequently asked questions
What is Priority Mail flat rate?
A fixed price for a specific box or envelope size. It stays the same regardless of weight up to 70 pounds or destination within the country, replacing weight and zone based pricing entirely for that shipment.
Is flat rate always cheaper than regular Priority Mail?
No. It saves money on heavy items going long distances. Light items going short distances are often cheaper under weight based pricing instead.
Are flat rate boxes free?
Yes. Every flat rate box and envelope is free at any post office or through USPS.com. No purchase is required.
What is the weight limit for flat rate shipping?
70 pounds, the same limit that applies to standard Priority Mail. The price stays fixed no matter how close to that limit you get.
Can I use my own box instead of a flat rate box?
No. Flat rate pricing only applies to official USPS flat rate boxes and envelopes. A similarly sized box of your own does not qualify.
What happens if my item does not fit in a flat rate box?
You need a larger flat rate size or regular weight based Priority Mail pricing instead. The box has to close properly without bulging for flat rate pricing to apply.
The bottom line
Priority Mail flat rate pricing removes weight and distance from the equation entirely, charging one fixed price per box size. That makes it a strong deal for heavy items going far. It is a poor one for light items going close.
Match your specific package to the right pricing model before shipping. See our full Priority Mail guide for the weight based alternative.
