I was playing around with cubic priority rates.
Here is one using an off the shelf USPS priority box.It's outside dimensions about 13.3 x 11.75 x 2.5
Comes out to a .3 cubic priority rate. With this box, it is cheaper to ship 1 or 2 lbs. regular priority. But from 3 lbs. to 20 lbs. can be shipped cubic priority for $11.39.
Another off the shelf USPS priority box, dimensions are about 15.5x12.5x3.25. Comes out to .4 cubic priority rate. Once again, 1 or 2 lbs. priority are cheaper, but to zone 8 3 to 20 lbs. can be shipped for $14.79
Anther example if you compare a large flat rate, 12x12x5.5, cost is $17.60 zones 1 -9. . If you use the same size box, up to 20 lbs. to zone 1 is only $8.07, cubic .5 priority rate. To zone 8, flat rate is $.31 cheaper.
Here is one using an off the shelf USPS priority box.It's outside dimensions about 13.3 x 11.75 x 2.5
Comes out to a .3 cubic priority rate. With this box, it is cheaper to ship 1 or 2 lbs. regular priority. But from 3 lbs. to 20 lbs. can be shipped cubic priority for $11.39.
Another off the shelf USPS priority box, dimensions are about 15.5x12.5x3.25. Comes out to .4 cubic priority rate. Once again, 1 or 2 lbs. priority are cheaper, but to zone 8 3 to 20 lbs. can be shipped for $14.79
Anther example if you compare a large flat rate, 12x12x5.5, cost is $17.60 zones 1 -9. . If you use the same size box, up to 20 lbs. to zone 1 is only $8.07, cubic .5 priority rate. To zone 8, flat rate is $.31 cheaper.