I have been on the phone and in chats regarding this issue and I can't seem to resolve it at all.
First thing is first. I have an AmEx tied to my account, I can purchase games with it on my Surface and on my XBox 360, that Amex also pays for my Gold Subscription every month. However, when I try to purchase something on my Windows Phone I only see a MasterCard and do not see my AmEx. I tried to get Microsoft to remove that MC from my account but they can't, they just keep telling me to wait until items on it clear. I haven't been able to remove it for months and it the balance is cleared, yet it says there is still a balance.
So I guess there's two things here.
1) I cannot see my AmEx for windows phone purchases. On my phone id does not display. If I go to windowsphone.com and try to purchase with my AmEx, I get an error saying "something went wrong". However, I can use that very same card with no issues on my Surface and Xbox 360.
2) I want to remove my MasterCard, there is no balance on it, and the card I want removed isn't even technically active anymore.
Let me know if more information is required. I'm beginning to become tired of talking about this.