Hello,
I’m having the same issue. We also have the premium version. When enabling the residential flag, on the debug, I see a <ResidentialAddressIndicator></ResidentialAddressIndicator> added to the <ShipTo> XML element. When disabling it, it just doesn’t appear. So I guess the plugin is sending the message correctly, but the API is not responding with a residential rate, but a commercial one. When I make a comparison with the value I get using UPS tool “WorldShip” I have a match for commercial rate, but not for residential (since the plugin always get the commercial).