set it as primary card on your google wallet setting.
or you can select the PB card before payment.

this is one of google technique to attract more customer to register for their google checkout account

and boost their sale on apps

and end up we might be purchase without thinking much

that's why this is their tactic
end up they might earn even more then previously due to ppls like us massively buying their paid apps
now i only can keep it 
who knows later on when not so popular the apps become free

which apps that is interesting with 10 cent tag?..i got soundhound, beautiful widget, reckless driving..