The Market Press USPS module does not work correctly.
First, it doesn't allow entering dimensions on products so that the correct Priority Mail or Express Mail box or envelope can be used for pricing the shipping.
This cannot be overcome in our particular situation by manipulating the shipping weight to select the correct package size for 2 reasons:
1. If you have MP set to put up to 1 pound shipment in the small flat rate box and anything over 1 pound in the medium flat rate box, the customer is presented with both the small flat rate box and medium flat rate box options. At a minimum, the USPS module should show ONLY the box for the shipping weight range specified and not give the customer a confusing choice which they wouldn't be asked if the USPS module worked correctly.
Please see this thread:
2. The "Priority Mail" and "Express Mail" options are always displayed no matter what weight is specified. These options are intended for the situation where the USPS provided flat-rate packing will not work and the customer provides the shipping box or envelope. They should only be shown IF the shipment doesn't qualify for any of the Priority Mail / Express Mail flat-rate USPS supplied packaging options. Even then, there's a problem: Even if you could specify the weight that requires customer supplied packaging, the without having dimensions, the USPS module cannot calculate correct shipping for that option.
We chose MP because it does a great job of displaying products with high-resolution images and dynamically resizing them for various screen sizes.
Unfortunately, we now regret choosing Market Press because of basic issues with shipping and payment modules.
Take a look at this page and resize your screen. You'll see how well MP handles the images: