Not sure what the point of your comment is (I've seen them in the thread I posted or some other related thread as well) but I think it's absolutely uncalled for.
I've bought and sold stuff online for some time, and this is a very strange error. There's actually not much info in the pre sales section regarding this - and what's suggested thus far have not been working for me (generating invoice, contact support, pay as a guest, use different credit cards etc). And I'll bet money that at least some other customers had not been able to purchase a licence as well.
So my point is that this is a legit issue that needs to be resolved and you have no business writing such a comment. If you're not a staff here, you're detracting from the professionalism of Xenforo Pre Sales support. If you are, F* you very much.
Regards,
Jack