USPS® offers six free "mailing boxes" that can be used with Priority Mail Cubic or weight-based Priority Mail:
Priority Mail Box - 1096L | OD: 9.25" 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: 13.68" x 12" x 2.875" | Cubic Tier 3
Priority Mail Box - 1095 | OD: 15" x 12" x 3.125" | 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" 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.
USPS discontinued Priority Mail Regional Rate Boxes on January 22, 2023, but you can use those boxes for Priority Mail Cubic shipments if you have old ones lying around!
* Measured on the Priority Mail Box 1097 from July 2013. Box sizes may vary based on the date they were printed/produced. We here at Pirate Ship always recommend measuring these boxes yourself to ensure you are using the most correct dimensions.
Need a box for Cubic Tier 5?
USPS doesn't provide any free mailing boxes compatible with Cubic Tier 5. However, if you're in search of a box around that size, you can order one from U-Haul here. The outer dimensions of this box are 12" x 12" x 6" 👍