What happens if there’s not enough gift card balance?

So, I had a Apple Store £1,100 gift card. I used it to buy 2x AirPods Max, but because one of them was going to be delivered in March I decided to order some other things instead and cancelled 1x AirPods Max with the other one due at the start of January. Problem is my purchases made my gift card balance fall £67 short of the £549 needed to buy the AirPods Max in January. There doesn’t seem to be an option to amend the payment options or add to the gift card to top it up. What will happen when Apple tries to take the payment from the gift card and there’s not enough?

thanks in advance!