Update January 22, 2023
USPS has discontinued Priority Mail Regional Rate. That's not a bad thing, because it was always cheaper to use a Priority Mail Mailing Box with Priority Mail Cubic or Weight-Based Priority Mail pricing instead 😉
You can still use any Regional Rate boxes you have on hand, but you'll need to weigh & measure it and only use it with Priority Mail, just like any other Priority Mail-branded packaging. We always recommend cutting your box down to size to save as much as possible!
Since USPS has discontinued the Regional Rate service as of 2023, the information below is no longer relevant, and is from 2022.
Historical 2022 information:
USPS Priority Mail Regional Rate boxes are specific kinds of packaging that you can get from the Post Office's website. To ship with these boxes, you have to buy the specific type of Regional Rate postage that matches the type of box you're using. You can see learn more and see the rates here.
It's a common misconception that Regional Rate is always a better deal than using your own packaging, but that's only true if you don't have access to Priority Mail Cubic. Shipping with Regional Rate is usually more expensive than if you used your own similarly-sized box, or a free "mailing box" from USPS.
Priority Mail Cubic is always cheaper than Regional Rate Box A
The Priority Mail Regional Rate A Box comes in two sizes:
Priority Mail Regional Rate Box A1 Box: 10 x 7 x 5 inches
Priority Mail Regional Rate Box A2 Box: 13 x 11 x 2.5 inches
Both of these boxes are just barely bigger than 0.2 cubic feet in volume.
If you used your own box which was *almost* the same size (smaller by even just a tenth of an inch like 10 x 7 x 4.9 inches or 13 x 11 x 2.4 inches), then the boxes would be in the 0.2 cubic feet pricing tier for Priority Mail Cubic, and would save you up to $1.28, depending on how far it's going:
USPS offers free "Mailing Boxes" you can use instead of Regional Rate A:
Use your own box for a custom size that fits your products, or USPS offers two free "mailing boxes" that are the cubic 0.2 size:
Priority Mail Box - 4 | 7" x 7" x 6" | Cubic Tier 2
Priority Mail Box - 1097 | 11-5/8" x 13-7/16" x 2-1/2" | Cubic Tier 2*
*Note: The Priority Mail Box - 1097 falls into Cubic Tier 2 when you measure it yourself, instead of inputting the measurements that USPS lists on the box (the ones listed above). If you're using this box, try taking your own measurements first!
Priority Mail Cubic is always cheaper than Regional Rate Box B
The Priority Mail Regional Rate B Box comes in two sizes:
Priority Mail Regional Rate Box B1 Box: 12.25 x 10.5 x 5.5 inches
Priority Mail Regional Rate Box B2 Box: 16.25 x 14.5 x 3 inches
Both of these are just slightly bigger than 0.4 cubic feet in volume.
If you used your own box that was just slightly smaller (like 12.25 x 10.5 x 5.4 inches or 16.25 x 14.5 x 2.9 inches), then they would be in the 0.4 cubic feet pricing tier of Priority Mail Cubic and could save you up to $1.35 depending on how far you're shipping it:
USPS offers free "Mailing Boxes" you can use instead of Regional Rate B:
Use your own box for a custom size that fits your products, or USPS offers two free "mailing boxes" that are the cubic 0.4 size:
Priority Mail Box - 1095 | 15-1/4" x 12-3/8" x 3" | Cubic Tier 4
Priority Mail Box - Shoe Box | 7-1/2" x 5-1/8" x 14-3/8" | Cubic Tier 4