I am using zen 1.3.9h and updated the module for USPS and get no options displayed at checkout. I turned the debug on and its getting info back from USPS but just not display any options. So after digging I found that if you offer only first class for orders <1lb the module will display nothing. If I choose to offer 2 options (first class and priority) it shows both.

Any idea why first class will not show on its own?