Eric -
Which version of the Cart are you running? Are you running the most current version? There are a number of fixes listed in our Change Log regarding USPS at the following link -
http://www.smith-consulting.com/Produ...
Not sure which modules you are comparing the Cart to but keep in mind that the Smith Cart module is the largest module available today in terms of number of tables, sprocs, dlls and ascx pages that need to be installed so your server environment/dnn portal needs to be tuned accordingly.
Unlike a rotator, blog or gallery module, the Cart is an ERP level system which requires ongoing maintenance as I have explained.
We are available and happy to help you resolve any issues you are having but if you are unwilling to take our recommendations or suggestions there's not much help that we can offer.
Hope this makes sense.