Medium Flat Rate Box Cost: Current Prices & Fees

The Medium Flat Rate Box is the mid-size option in the Priority Mail Flat Rate lineup. USPS charges one set price for it no matter how far the package travels or how much the contents weigh, within the weight cap for flat rate packaging. That makes it a popular pick for dense items going a long way, such as books, canned goods, tools or a stack of folded clothing.

Prices on this page come from the USPS Notice 123 price list and the USPS Priority Mail flat rate page, checked on Oct 11, 2026. USPS updates its prices twice a year, so confirm the figure at the Post Office counter or on usps.com before you ship.

The box has two prices, depending on how you pay for postage. The Post Office counter charges the retail price. Buying and printing a label online through USPS Click-N-Ship or a shipping platform gets you the commercial price, which is lower.

The online discount applies to the postage itself. If you buy a label online and then hand the box to a carrier or drop it in a collection box, you keep the lower price. Counter service costs more because a clerk handles the transaction, and the retail price also covers walk-in convenience for people who do not want to print anything at home.

USPS offers two medium box styles: a top-loading box and a side-loading box. They hold roughly the same volume but open differently, which affects how easily you can pack long, flat items. Check the exact inside dimensions printed on the box or listed on usps.com, because a package that bulges or will not close is not eligible for the flat rate.

Good candidates include:

• Books, DVDs and board games
• Shoes in a shoebox, if the shoebox itself fits
• A few pairs of jeans or sweaters packed tightly
• Small kitchen appliances, tools or hardware
• Care packages with a mix of snacks and household goods

Poor candidates are light, bulky items such as a single pillow or a stuffed toy. They fill the box without weighing much, and a regular Priority Mail or Ground Advantage package would likely cost less.

Flat rate pricing helps when the weight is high or the distance is long. A package that travels across the country can cost much more by weight and zone than the flat price, and the box removes that guesswork. A short trip within your own region with a light package often costs less on a weight-based service.

A simple way to test it: weigh your packed parcel, measure it, and compare the flat rate against the weight-based quote for the same destination. The USPS postage price calculator shows both. Remember that the flat rate box does not charge extra for weight, so the heavier the contents, the better the deal gets.

Size Rules That Catch People Out

The box has to close properly with the flaps sealed. Adding tape to hold bulging contents, or building an outer cardboard layer, can void the flat rate and lead to postage due. USPS also expects you to use the official box as supplied. A homemade box covered with Priority Mail stickers is not eligible, even if the dimensions match.

Most people choose between the small box, the large box and the envelopes. Here is how the retail prices line up.

If your items fit in the small box, you save by using it. If you are squeezing things into the medium box with no room to spare, stepping up to the large box may be worth it, since overstuffing can mean the box will not seal. For documents and flat items, the envelope options cost less than any box.

USPS supplies Priority Mail flat rate boxes at no charge. You can pick them up at a Post Office, ask your letter carrier to leave a stack, or order them on usps.com for delivery to your door. Online orders are handy if you ship often, since you can stock up and skip the counter trip.

Boxes are meant for Priority Mail shipping only. Using a USPS-branded box for another carrier or another mail class is prohibited.

• Pay online: The commercial price is lower than counter pricing for the same box.
• Pack tightly: Because weight does not change the price, fill gaps with soft items and use paper to prevent shifting.
• Compare services: Run the same parcel through a weight-based service before committing.
• Check insurance: Priority Mail includes some built-in coverage, but higher-value items may need extra. Details are on the USPS site.
• Watch for price changes: If a USPS increase is coming, boxes you already own still work; only postage changes.