Weight-based Priority Mail® is the cheapest way to ship large packages

Unlike other places to buy USPS postage online, Pirate Ship passes through Commercial Pricing for Priority Mail with absolutely no markup, fees, or monthly costs. So you get the deepest discounts available for free, with no risks or commitments. 

Packages exceeding 1 cubic foot are charged the dimensional weight (Length x Width x Height / 166) if greater than the actual weight.

Dimensional weight is common for every carrier, basically because they don't want to fill up their trucks and planes with packages that are large but mostly empty & light.

If your package is larger than 1 cubic foot (for example, if the dimensions are 13" x 13" x 13") then USPS uses a formula to calculate the minimum rate they will charge. It's based off the dimensions, which is why this is known as the "dimensional weight" or "DIM weight". If your package's dimensional weight is "heavier" than the actual weight, you will be charged as if your package were that heavier weight.

Dimensional weight is calculated by multiplying a package's length, width, and height and then dividing by the volumetric divisor. USPS uses 166 for their divisor.

So for an example of a package that measures 13" x 13" x 13", you would multiply the three dimensions to get 2197, and then divide that by 166 to get 13.23, which is the dimensional weight. So if your package's actual weight was less than 13.23 pounds, you would get charged the 14 pound Priority Mail rate. If your package was more than 13.23 pounds, the dimensional weight would be disregarded and you would just be charged the actual weight.