I have 8.1 Pro installed and am wanting to spend the extra $12.99 to get the media center pack. 

Nothing I do will let me get the purchase completed. It always throws an error saying they couldn't process the order and to check my details. 

Needless to say I've checked my details. 

I've also checked all my store payment details, purchased apps, checked my timezone details, reset routers, changed dns settings etc etc.

I called MS support and spent half an hour talking/being transferred to people in the US/Canada and they came to the conclusion that (without asking me anything but my name) I was going to have to buy a new 8.1 Pro licence. I hung up not long after that. 

I have a media center key I got when they were giving them out for free so I figured i'd try that, even though I had to have used it before Jan 2013. It installed and worked fine until it threw a fit and made my 8.1 pro install deactivate and made it unable to be activated again.

Anyone able to tell me what I need to do to get this? I'd be willing to buy a key from someone else if I have to but you can't buy them anywhere but on the damn add features app in windows.