As long as you are buying the album for yourself (i.e. not gifting) then your balance should be used up first - from http://support.apple.com/kb/HT5582 :
When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.
Asking for credit card details is a way of confirming that you are able to use that country's store e.g. you aren't trying to use a different country's store.
Card details can also be required depending upon how the account was created e.g. if the instructions on this page weren't used when creating the account then card details will need to be entered before it can be used to buy any item : http://support.apple.com/kb/HT2534