Random thought. So you said you can make one payment and when attempting to make a second or third payment you get the 406 error? I don't know if it could possibly have anything to do with the fact that your computer has information stored as temp files indicating that you've already made a successful payment, which the site could check for when you attempt to make a second payment, and if the site sees those temporary files on your pc it would understand to decline a second purchase, but if that's the case, which it's probably not so don't get your hopes up...you could try making a payment, close your browser, use a program like Ccleaner to clear the cache on your computer, or alternatively use your internet browser options to remove temporary internet files, and then open your browser back up, go to the Mogstation, and attempt to make another payment. Like I said, it was just a random thought, but if it works...well, that would be pretty cool, and it would save you a lot of hassle.