Can't purchase Microsoft Flight Simulator Standard version using my Microsoft Account balance

Recently, I wanted to spend my $70 that I have in my Microsoft Account to buy the new Microsoft Flight Simulator (Standard) that costs $59.99 with taxes (so about $65 total). The problem is that it won't let me spend my MS account balance as it asks me how would I like to pay the rest. So, I had $60 because I've redeemed 60 promo codes from Rockstar Energy (I added $10 by redeeming a gift card through making an online purchase so I can cover the remaining cost). I didn't know that the promo funds would expire until I received an email last month saying that they will expire by May 9 (which is today). I have refreshed the tab where it displays my account information, including the MS account balance, only to see that the expiration date has changed to June 9. I've tried contacting Microsoft support and discussing the issue with one of the agents. However, they have told me it's best to contact Xbox technical support but they didn't help me much either. I still cannot use my MS account balance to pay for the game even though there are sufficient funds as it seems that the promo funds haven't expired. Can someone please help me?

:)