The Xbox Store is currently offering a 3-month ultimate Game Pass for 1 euro (after that 12.99 euro). I have 9.01 euro in credit on my Xbox account. When I go to order, it shows me whether I want to pay the amount with my remaining balance. If I click on "Yes, order" I will be redirected to the "Select a payment option" task. You can then redeem it via PayPal, mobile phone and voucher card. I'm desperate right now 😅
I've already written to customer support, but I've been waiting for it for two days.
Call support, had the same problem and the support guy somehow did it with my credit.