A few months ago I had some problems with the debit card I use on the net and for some reason I wasn't able to use it with Namecheap. I contacted support and I got very fast replies, but they couldn't actually help because the use a third party to process payments and as such they didn't know the actual reason why the charge kept being denied.
I then tried registering the domain with Godaddy, using the same card. It didn't work with them either. Proof that Namecheap was not to blame at all! As such, I decided to used another card, and since the order was almost done with Godaddy, I registered the domain with them. Everything works OK. So far I had no reason to contact their support though, so I cannot comment on that.
This post remind me. I really should contact my bank about that non working card.