While I don't represent Oasis I do have some insight on this. It isn't uncommon for credit card's to put on a temporary block for a purchase that appears to be made to an over seas organization. While I was over seas I experienced this same problem. (And actually when I came back to america I had the same thing happen all over again because of the change of geography)

Anyway, an easy fix to this would be either to talk to your credit card company (Or bank) OR you c*e pay pal. Pay Pal offers you the ability to spend without having to worry about this temporary block that comes with the use of credit or debit cards, even with purchases to an over seas organization.

I hope this helps, and good luck. It's worth noting that if you don't have a pay pal, its actually really simple to make one so don't be intimidated by the process. Takes like 5 minutes.