USPS provides these envelopes free of purchase. When using this postal option you'll want to use USPS's special Flat Rate Envelopes. Legal Flat Rate Envelope (roughly 15 X 9) What is a Flat Rate Envelope? Padded Flat Rate Envelope (roughly 9 X 12) Standard Flat Rate Envelope (roughly 12 X 9) And when shipping packages, select First Class Large Package.Order Flat Rate Envelopes from USPS for FREE now: We suggest you review your shipping process to keep your larger packages in the smallest box possible to avoid these surcharges. * 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. * Calculating the cubic dimensions of your packages isn't difficult. And when shipping packages, select Large Package. * Does not apply to First Class and Media Mail.

Packages with missing or incorrect package dimensions that result in any dimensions-based fees, a $1.50 surcharge will be applied.If the volume of the package is greater than 2 cubic feet, a $15.00 surcharge will be applied*.If the longest side of a package is greater than 30 inches, a $15.00 surcharge will be applied.If the longest side of a package is more than 22 inches but less than 30 inches, a $4.00 surcharge will be applied.

* Starting April 3, 2022, the USPS is adding new surcharges for large packages*:

Flat rate mailing envelope