Payment at app store
Download of app from store always requires just payment by card even though I have enough credit.
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When you make a purchase in the App Store, it always shows your credit card as if it is deducting the money from there, but if there is money on your Apple Balance, then the money is always paid from it first. If there is not enough, then the remaining amount will be deducted from your Credit Card.
Find more useful information here:
Add money to your Apple Account balance - Apple Support