Apple ID Funds versus Apple Pay

Here is my situation. I would like to pay for a potential MacBook Air using my Apple ID funds (have traded in previous MacBook Air for a credit) - but I believe that during the check-out process my only two options are 1. Personal Credit Card Account or 2. Apple Pay (Not Apple ID) My questions, I suppose are:

1. How do I transfer my Apple ID funds to the Apple Pay account so that I might check-out successfully?
2. If it is not possible to do so - is there another way for me to be able to pay for my product using my credits/funds that are in my Apple ID account? (Hope that makes sense?) Super thankful for anyone’s advice/support. Happy Friday!