The method used to pay is fully automated: How iTunes Store purchases are billed - https://support.apple.com/HT201359 - When you make a purchase, 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." You can't change the payment type order. For details on how an item was paid: See your purchase history in the iTunes Store on a Mac or PC - http://support.apple.com/HT204088
Note that some countries do not accept debit cards, or may require automatically renewing subscriptions to be charged to a credit card instead of account balance.