PSA: Never, ever attempt to buy an iPhone on pre-order day with an accumulated gift card balance.
I was there, as I am every year, refreshing the store at opening time but this year I had over £1000 in Apple credit to use. I applied my balance to the cost of my chosen iPhone 16 Pro Max, got to the 'Place Order' button, clicked it and then got an error message saying payment wasn't authorised by my credit card/bank for the portion not covered by the credit.
So I tried again and again and got the same thing. Then, I thought, I'll just buy some more credit so my total credit equals the entire cost of the phone. So I did that and tried again, all the while knowing that ship dates would be starting to slip.
Clicked the place order button again and got the same thing. £1199 in store credit utterly useless. Why am I getting messages about my credit card/bank not authorising when the issue is not with them (I even checked this by ordering a phone successfully with my credit card and then cancelling)?
So with ship dates now having slipped into October I then spent the next 4 hours speaking to eight different people, only the last of whom actually took some ownership of my problem and promised to get it sorted and call me back on Monday. Each chirpy "I'd be so happy to help you today!" became increasingly like having my ears grated. By the time I'd finished explaining it for the umpteenth time, the words had lost all meaning; they were just noises coming out of my mouth.
This morning I discovered a thread on the Apple Community forums where someone (among the many who have had exactly the same issue over the years) suggested that a method that works is placing the order with a credit card and then calling Apple to get the post-sales department to change the payment method to your Apple account balance.
Bloody hell - it actually worked, and I now have an iPhone ordered but it won't be delivered until mid-October now. First time I've missed an iPhone launch since 2007 (I know; teeny tiny violins for me)
But, Jesus, the tortuous process to get it sorted was beyond painful. It's nuts that this is a recurring problem and Apple hasn't fixed it, or even told its support staff what the solution is.