Ok, It's quite clear why that is happening. The package error for first class is informational only. USPS does not provide first class domestic for items over 13oz so the module behaviour is correct in this case.
Use the USPS online documentation to find out what limitations apply for a particular service.
Also you are filtering based on weight/dimensions and did not provide any weight dimension rules to get it going.
Look here: http://postcalc.usps.gov/
for some dimensions of flat rate boxes and set some dimensions in the configuration.
Module weight/dimensions config example:
Code:
8x5x1.5;0.5
11x8x5;3
12x12x5;6
regular;+
This configuration will quote the Small FR box for items 0.5lb or less.
Medium FR for items 3lb or less
Large FR for items 6lb or under
All Flat rate options are filtered once weight is over 6lb
You can use this and customise the weights to your liking.
You can see how the weight/dimensions filtering and dimension selection works in debug mode. Your examples show dimensions being set to 'regular' which is 12x12x12 filtering out all flat rate options.