Priority Mail Cubic is the best kept secret in the shipping industry, but it's not just for boxes... you can get dimension-based rates with poly or padded envelopes as well, as long as they're not more than 18" in either direction.
USPS calls these “Soft Pack Cubic Rates.” Just like regular Priority Mail Cubic, the rates are only based on dimensions and your package can weigh up to 20 lbs. This can save you up to 88% off what you'd pay with the weight-based rate. The only difference for envelopes is the way the cubic pricing tier is calculated:
Measure the length and width of the empty envelope or softpack mailer before you put anything into it (in inches);
If either of the measurements exceed the nearest quarter inch, round down to the nearest 1/4 inch (for example, 8.6" inches turns into 8.5" inches);
Add the two measurements together for the total and use the chart below to determine which pricing tier your softpack envelope is in (note that the total cannot exceed 36").
Priority Mail Cubic Softpack Commercial Plus Pricing Tiers:
0.1 - Envelopes with a Length plus Width measuring more than 0" up to 21"
0.2 - Envelopes with a Length plus Width measuring more than 21" up to 27"
0.3 - Envelopes with a Length plus Width measuring more than 27" up to 31"
0.4 - Envelopes with a Length plus Width measuring more than 31" up to 34"
0.5 - Envelopes with a Length plus Width measuring more than 34" up to 36"
Use the original dimensions of the empty envelope & don't fold/tape it down!
To qualify for Priority Mail Cubic Softpack, your envelope cannot be folded over and taped down. USPS requires you to use the original dimensions of the empty envelope, so don't fold it over / tape it down so that it's smaller. If your envelope is too large for the items, we recommend cutting it down to size before using it.
You can't use expandable (pleated) envelopes
Note that if your envelope is expandable (pleated), technically you have a 3rd dimension just like if it were a box, so you'll need to use the standard method of determining your Cubic pricing tier.
This is what an expandable envelope looks like:
If you're using one of these, you need to select "Box" as your package type and enter all 3 dimensions (Length, Width, and Height).
You can put a box into the envelope, as long as it's still shaped like an envelope!
USPS only cares about your outer packaging, so if there is a box inside your poly mailer or envelope, that's no problem. But you can't "shrink wrap" or tightly tape a bag or envelope around a box and call it an envelope. If it clearly has 3 dimensions, it is a box and you need to enter all 3 dimensions when purchasing postage.
USPS Priority Mail Cubic Softpack shipping rates
Here's the current Priorty Mail Cubic Softpack rates.
Pirate Ship passes through the cheapest USPS rates available for free, with no markup, fees, or minimum volume requirements.