I've had a few sites act up like that and traced the problem down to my HOSTS file (I use it to try and block most tracking cookies, works quite well except for a few sites that really want to track you). Try disabling your HOSTS file and see if that doesn't let you complete the purchase. Just rename it, adding an extension to it works. Then just rename it back when done.
And if you're using AdBlock, disable it also for the site, at least until you're done.