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You have to use the branded Flat Rate boxes to get the Flat Rate shipping price, and you can order them online or pick them up at your local Post Office. Order free Flat Rate boxes from the USPS.To ship with this service, follow these easy steps: The USPS Priority Mail Flat Rate service will ship your package anywhere in the U.S. You can easily ship like the experts just know the USPS Flat Rate box sizes, prices, and how it works before you buy. And when shipping packages, select First Class Large Package.USPS Priority Mail Flat Rate boxes are an easy-to-use, cost-effective shipping option. We suggest you review your shipping process to keep your larger packages in the smallest box possible to avoid these surcharges.
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* Starting April 3, 2022, the USPS is adding new surcharges for large packages: That is the exact cubic dimension of your package. Multiply the Length, Width, and Height, and then divide that total by 1,728. For boxes that aren't square, measure the Length, Width, and Height of your box in inches. A 2 cubic foot square box is pretty big and well beyond the size of most packages being shipped. A square package 2 feet by 2 feet is 2 Cublic feet. A square package 1 foot by 1 foot is 1 cubic foot.
![priority padded flat rate envelope cost priority padded flat rate envelope cost](https://www.usps.com/ecp/asset/images/EP_14_F-L0.jpg)
* Calculating the cubic dimensions of your packages isn't difficult. And when shipping packages, select Large Package.
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* Does not apply to First Class and Media Mail.
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