Calculated Shipping - USPS Flat Rate Options

I have upgraded to the premium version of Marketpress for the Local Pickup option. When configuring the USPS settings, I tried to choose the flat rate boxes (and have the dimensions and weight in the Standard Boxes and Weight Limits), but when I attempt to check out it is giving the option for ALL of the flat rate boxes. This allows my customers to select "Small Flat Rate Box" when they need the Medium or Large.

I have weights set on both the shipping settings and the products, so it would seem that the Small Flat Rate box option would not appear if the order weight is higher than the setting in shipping options.

Any ideas?