I can relate why you are so frustrated... I have had $14 on my account for the past 3 years now & also cannot make a purchase from the microsoft store.
This has happened to me twice now, first I wanted a simple utility that was on sale for $6.49 and I emailed & complained and the CSR replied saying they had never heard of having funds on a microsoft account, I was furious & eventually just let it go, it seems
they just wont answer the question why this happens.
Now just in the past few days I tried again to get "Bridge Constructor" for my daughter, the demo worked fine & she liked it but again It has no option to use my funds in my account.
For the record I have tried various different regions namely the US and Canada and even navigated right from the microsoft store application and clicked the link right next to the balance, but no matter what I do there is NEVER an option to pay with my account
balance...
Can someone please help complete the transaction, or just refund the money to my bank, I will even give you the balance left after purchase of the game which is $3.49 CA ... I will let you keep some of it but please don't steal it all from me & not let me
get anything that's not really fair