USPS® offers six free "mailing boxes" that can be used with Priority Mail Cubic® or weight-based Priority Mail®:

Priority Mail Box - 1096L | OD: 9-7/16" x 6-7/16" x 2-3/16" | Cubic Tier 1
Priority Mail Box - 4 | OD: 7-1/4" x 7-1/4" x 6-1/2" | Cubic Tier 2
Priority Mail Box - 1097 | OD: 11.625" x 2.5" x 13.4375" | Cubic Tier 3
Priority Mail Box - 1092 | OD: 12.25" x 2.875" x 13.6875" | Cubic Tier 3
Priority Mail Box - 1095 | OD: 12.5" x 3.125 x 15.375" | Cubic Tier 4
Priority Mail Box - Shoe Box | OD: 7-3/8" x 5-1/4" x 14-7/8" | Cubic Tier 4

You can order these in bulk from USPS.com/supplies.

Keep in mind that you MUST purchased Priority Mail® postage to use these boxes (you cannot use them for First Class Package® or any other service).

Also note that any USPS® package that says "Flat Rate" or "Regional Rate" on it requires that you buy the corresponding postage for that package type; you cannot use those boxes for Priority Mail Cubic® or weight-based Priority Mail®.

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