Development is aware of the UPS rate issue and looking into it; however, since we just submit the weight and origin/destination zipcodes to UPS and apply whatever rate they return, this may not be something we have any leeway to address, if UPS is showing you one rate on their Web site and returning a different rate to our rate-lookup queries for the same weight and zipcodes.
You might want to verify that at least your Shipping Origin zipcode is accurate in your Seller Admin > Cart Shipping Settings. If you do change the zipcode there, you can click Next to save that change and then on the next screen just click Back to Admin.