Alrighty,
The Support Team suggests that you clear your internet browser's cookies, and then reattempt the purchase.
You can find instructions on clearing your internet browser's cookies
here:
http://www.wikihow.com/Clear-Your-Browser's-Cookies
If that does not work, we also suggest switching to another internet browser in order to complete the transaction.
If you are using a mobile browser, please download the Chrome App and complete your purchase using that.
If the problem still persists, please provide the MC account name and we'll look into the issue.
If you have any further questions, feel free to contact me through PM. Have a pleasant stay on the MCGamer Network and take care!
TL.