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. 

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 194 for their divisor, which is much more competitive than the other carriers. Even more competitively, USPS only applies dimensional weight pricing to packages in zones 5-9.

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 194 to get 11.32, which is the dimensional weight. So if your package's actual weight was less than 11.32 pounds, you would get charged the 12 pound Priority Mail rate for zones 5-9. If your package was more than 11.32 pounds, the dimensional weight would be disregarded and you would just be charged the actual weight.

Dimensional Weight can make it more expensive to use Priority Mail for a large box than it would be if you used Parcel Select Ground®, but Priority Mail is a much faster service and includes $100 of insurance.