USPS plugin Priority flat rates incorrect

Hello,

I am trying to setup the USPS plugin for use with my site. I have selected “Use offline rates” and have turned on “Flat rate services.” I intend to ship using the USPS flat-rate priority boxes (small, medium, and large). I have also defined product dimensions and weights for all of my products.

However, when I have tested the shipping rates in my store, the calculated rates are inaccurate and do not increase based on the product sizes. For example, I have a soap bar that is 2.75 x 2.125 x 0.75″, which is 4.383 cubic inches. A small flat-rate priority box is 8.625 x 5.375 x 1.625″, which is 75.333 cubic inches. The price of this small box on the USPS website is $5.95 up to 70 lbs.

So I should be able to fit 17 of these soap bars into one of these boxes (75.333/4.383 = 17.18). Any number over 17 should trigger either a second box or the larger box size, in my understanding.

The problem is that I have to increase the quantity to 27 soaps bars to trigger any increase in price. The plugin right now shows $6.31 (with tax) for shipping for all quantities of this soap bar up to 27. Thus, I would have to use a larger box or multiple boxes and eat that shipping cost.

Can someone please help me to configure this plugin correctly since the automatic box packer seems to be incorrect? Thank you in advance.