Did you check, if the payment info for your credit card is also complete, including the delivery address?
Since you will be charged for your MacBook Air after it has been delivered, which at this point reduces the amount of money in your Apple Account, your credit card will be charged for the remaining amount for your new iPhone purchase.
The combination Apple Account B balance + Credit Card might cause this issue as well.
Did you already call the Apple Online Store Support? It's in their interest to sell Apple products, so they should be able to help you sort that out.
...
You can use up to eight Apple Gift Cards at apple.com or when placing an order over the phone at 1‑800‑MY‑APPLE. You can use up to eight Apple Gift Cards for one transaction when you purchase at an Apple Store. You can use your Apple Gift Card(s) with an Apple Account Balance and Apple Pay or one credit card. Apple Gift Cards cannot be used to purchase another Apple Gift Card.
...
copied from: Payment, Financing, Refunds and VAT - Shopping Help - Apple