FedEx One Rate and USPS Priority Mail Flat Rate both let you ship a heavy item for a price set by the box rather than the scale, but they are built on different services: One Rate is a flat price for FedEx Express services (overnight and 2 to 3 day options) up to 50 pounds, priced in distance bands, while Priority Mail Flat Rate is a single nationwide price for USPS's 1 to 3 day product up to 70 pounds. If you need a guaranteed express delivery, One Rate is the flat rate that fits. If you need the heaviest, farthest parcel at one predictable price, Priority Mail Flat Rate usually is.

How does FedEx One Rate work?

FedEx One Rate is a pricing option layered on top of FedEx Express services. According to FedEx:

Services. One Rate applies to FedEx First Overnight, Priority Overnight, Standard Overnight, 2Day A.M., 2Day and Express Saver. It does not apply to FedEx Ground or Home Delivery.
Packaging. You must use FedEx branded packaging: the FedEx Envelope, Pak, Small Box, Medium Box, Large Box, Extra Large Box or Tube.
Weight cap. Up to 50 pounds per package, with the Envelope capped at 10 pounds.
Pricing. Flat by packaging type and service, within three distance bands (local, regional and national). It is not one price for the whole country.
Coverage. FedEx includes up to $100 of declared value; more can be purchased.
Guarantee. Express services carry the FedEx money back guarantee on delivery time, subject to FedEx terms.

The practical effect is that a dense 40 pound box in a FedEx Medium Box ships express at a known price with no dimensional weight surprise, because the box is the price.

How does Priority Mail Flat Rate work?

USPS Priority Mail Flat Rate is a pricing option on Priority Mail:

Service. Priority Mail, quoted at 1 to 3 business days, not guaranteed.
Packaging. USPS supplied Flat Rate Envelope, Padded Flat Rate Envelope, Legal Flat Rate Envelope, Small Flat Rate Box, Medium Flat Rate Box (two shapes) and Large Flat Rate Box. The packaging is free from USPS.
Weight cap. Up to 70 pounds in any flat rate container.
Pricing. One price per container nationwide. Zone does not matter.
Coverage. USPS includes up to $100 of insurance against loss or damage.

The Medium Flat Rate Box measures 11 1/4 x 8 3/4 x 6 inches (or 14 x 12 x 3 1/2 in the alternate shape), and the Large Flat Rate Box measures 12 1/4 x 12 1/4 x 6 inches, per USPS. If it fits and closes with the flaps flat, it ships.

Where do the two diverge?

Speed and guarantee. One Rate is express with a money back guarantee. Flat Rate is a 1 to 3 day estimate with no guarantee. If the arrival date is contractual, that alone decides it.

Weight. Flat Rate carries up to 70 pounds; One Rate stops at 50. A 60 pound box of books has one flat rate option, and it is USPS.

Distance. Flat Rate is truly national at one price. One Rate steps up across local, regional and national bands, so a coast to coast One Rate shipment costs more than a cross town one in the same box.

Packaging. Both require carrier branded containers, and both supply them. The FedEx Extra Large Box and Tube have no USPS flat rate equivalent, so a long or very large item may only have a One Rate option, or none at all.

Delivery footprint. USPS reaches PO Boxes and APO addresses. FedEx Express does not deliver to PO Boxes.

Weekend delivery. USPS Priority Mail delivers on Saturday at no extra charge in most locations. FedEx Express Saturday delivery is an option with its own terms.

A worked example

You ship a 30 pound box of cast iron cookware from Ohio to California. The set fits in a USPS Large Flat Rate Box and also in a FedEx Large Box.

Check the weight caps. 30 pounds is under both the 50 pound One Rate cap and the 70 pound Flat Rate cap.
Check dimensional weight. Neither flat rate option uses it, which is the whole point for a heavy dense box. On a regular weight based FedEx Ground or UPS Ground label, the 12 x 12 x 6 box would be 864 cubic inches, which divides by 139 to about 6.2 pounds, so it would rate at the actual 30 pounds anyway.
Check the distance band. Ohio to California is national for One Rate, the most expensive band. For USPS Flat Rate the zone is irrelevant.
Check the promise. If the customer bought standard shipping, the Priority Mail Large Flat Rate Box is the candidate to compare against weight based ground rates. If the customer needs it in two days with a guarantee, FedEx 2Day One Rate in the Large Box is the candidate.
Compare. Quote the flat rate options next to weight based USPS, UPS, FedEx and DHL rates and pick the lowest that meets the deadline.

The point of the example is that the two flat rate programs rarely compete head to head. One answers "express, predictable, under 50 pounds." The other answers "heavy, far, no rush, under 70 pounds." Weight based ground rates are the real competitor to both, which is why comparing everything on one screen matters. Our flat rate vs weight based shipping page covers when the flat price loses to a weight based one.

How should you decide per order?

Guaranteed delivery date required: FedEx One Rate in the smallest FedEx box that fits, or another express service.
Over 50 pounds: Priority Mail Flat Rate is the only flat rate option; compare it with UPS or FedEx Ground.
Heavy, dense, going far, no deadline: Priority Mail Flat Rate is the first thing to check.
Light or going a short distance: neither flat rate program is likely to win; a weight based rate usually does.
Item value over $100: add declared value or insurance on either carrier.

GoatLabels shows the USPS flat rate prices you qualify for and the FedEx services side by side with UPS and DHL for every order, with no monthly fee and a prepaid wallet that pays per label, as described on the pricing page. Orders from Shopify, Amazon, eBay, Etsy, Walmart and nine other connected stores land in one "To Ship" queue, and the AI assistant can draft the label from an order screenshot if you are not on a connector yet. Sign up to run your heaviest product through both programs, and check the FAQ for how flat rate packaging is selected in the app.

Quick answers

Is FedEx One Rate a ground service? No, One Rate applies only to FedEx Express services such as 2Day and Standard Overnight, not to FedEx Ground or Home Delivery.

What is the weight limit on each? FedEx One Rate accepts up to 50 pounds per package, while USPS Priority Mail Flat Rate accepts up to 70 pounds.

Is One Rate the same price everywhere? No, FedEx prices One Rate in local, regional and national distance bands, whereas Priority Mail Flat Rate is one price nationwide.

Do I have to use the carrier's box? Yes, both programs require carrier supplied packaging, and both carriers provide it at no charge.