I always use a Disney Gift Card. When it’s time to pay, select gift card as your payment type. You will have to enter the card number each time, and some people have saved their card number as a note in their phone so they can copy and paste the number. I just enter it each time. The screen will give you an updated gift card total, which I usually screen capture to track my spending.
The default is going to be your credit card on file. You have to specifically choose to pay by gift card.