Yes, there is a problem with paying through AlertPay - It is already known. Dave is on the verge of fixing it soon.
You could try instead to pay using an American Express Card instead - since that should work judging by the error message.
You can also pay through using a prepaid card... Get a prepaid card of $10 and PM Dave. https://www.mpgh.net/forum/private.php?do=newpm&u=1
Or you can rack up 250+ posts and pay through PayPal... Those are your own Payment Methods. AlertPay is down for some people... So those are the only alternatives. It'll be fixed soon, don't worry.
~Closed.