How can I recover a used Apple Gift Card balance after failed order?

I've just tried to purchase a MacBook and the free air pods for university with an apple gift card which has a value of £150, I tried to make the purchase including £7 for shipping pre noon which totaled £2 over my current bank balance (i noticed that the apple pay showed the full sum including the money that would be covered by the gift card), but i assumed since i was using the gift card that i would only be charged for the remaining sum of approx £785. i make my order and the charge on my card (785) is declined and i'm told that apple is waiting for me to send them the money and when i contact a specialist i'm told to speak to my bank to figure out what went wrong in the transaction. the problem i have is that i thought it would be easier to simply cancel the order, since i wouldn't be charged if i did so as the money hadn't gone through. i cancel my order, but now when i go back to make the purchase again, this time without the shipping so it should be within my bank balance the gift card charge is now zero. what do i do!! I could simply pay without the gift ard but it was a present from my dad and £150 i a lot of money.

please help!!!

[Re-Titled by Moderator]

Original Title: Gift card balance now zero, please help!!!