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

Priority Mail Box - 1096L | OD: 9.25" x 6.25" x 2" | Cubic Tier 1
Priority Mail Box - 4 | OD: 7.25" x 7.25" x 6.5" | Cubic Tier 2
Priority Mail Box - 1097 | OD: 13" x 11.25" x 2.25" | Cubic Tier 2
Priority Mail Box - 1092 | OD: 12.25" x 2.75" x 13.5" | Cubic Tier 3
Priority Mail Box - 1095 | OD: 12.5" x 3 x 15.25" | Cubic Tier 4
Priority Mail Box - Shoe Box | OD: 7.25" x 5.25" x 14.75" | Cubic Tier 4

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

Keep in mind that you MUST purchase 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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