I’ve had similar problems today, Apple Store app not live until 1.05pm, then my Apple Pay wouldn’t go through. Had to quit the app and head back to my basket. It worked then and I got a delivery date of the 13th. About an hour ago, received the text from Santander to confirm my transaction. Replied to it to confirm. Then half an hour ago, App Store notification to say payment declined and to contact my bank or change payment method. On the order page it looks fine, so I think Apple will try again and bank should now allow it? Massive phone queues at my bank and at Apple, so I will just wait until the morning and check the order page again.