I eventually was able to pay the invoice online, but when I got to the final step the page came up as "not available" again. The receipt sent to my email also was the same thing, a copy of a page that said "not available". When I clicked "back" though, I found the receipt that showed my charge card HAD been accepted. Odd. I guess I just have to wait to see if the charge really shows up on my VISA in the next couple days to know for sure if it went through or not.