Keep getting the "Your purchase cannot be completed at this time. Please try again." error ...

All I did was skim this but it might be helpful to you if you do want to explore my suggestion:
www.apovpn.com/byod-or-bring-your-own-device

I have never been able to buy content with my xbox overseas. Im in the Navy and bring it with me underway. I believe the marketplace is restricted to US IP addresses. Same thing happens to me when buying digital content via steam or amazon too. Just stick
to buying physical discs and having them mailed when you are overseas. Everynow and then arcade games also come on disc, those would work.

Thanks alot. Strong VPN was what i tried out, and it connected me as if I was in LA. Worked out perfectly! Appreciate all the help!

And yes, I was able to purchase via VPN from my computer and download on my xbox via overseas IP.

Pay for a VPN service that you can get a router to work with (so you can pass your XBox traffic through it). I've never used any but I know some that use Strong VPN. I have the same problem (am overseas with an APO). The problem is your overseas IP, not
your billing address. A friend of mine flashed a router he bought with DD-WRT (www.dd-wrt.com/.../index) and Strong VPN uses a VPN setup that you can configure in DD-WRT.

If you aren't technologically inclined this is probably far more than you'd be interested in trying. If this seems like too much you can get an Amazon Prime account for free priority shipping to your APO (or pay shipping) and then just buy all your stuff
through it (and waiting the 10-16 priority days).

VPN services can be as cheap at $6-8. They can also help you for things like Amazon Instant Video, Netflix, Hulu, UK content etc.

You *might* be able to get away with making the purchase on a US IP and then downloading from an overseas one. A friend of mine only tested a US IP and had the download routed through the US IP.