I once had a problem where the Kobo website simply stopped accepting my credit card. There was no problem with the card itself; my credit card company said that they were not even getting authorisation requests for the charges, so the problem has to have been entirely internal to Kobo. Dealing with Kobo customer care via email was, predictably, akin to bashing my head against a brick wall until bloody. I eventually tried phone support and spoke with a young man who seemed bright and capable and immediately understood my problem and agreed that it must be caused by something within Kobo's systems, and he turned out to be quite unable to get anything at all done about it.
I eventually gave up. When I tried again a few weeks later, Kobo were willing to take my money again.