[entwisi]

If they did not make clear that credit cards are not accepted then they have to take it. You are making a legal offer of payment in a format that they accept(you might need to double check their T&Cs in detail to ensure tha it doesn't explicitly state not). Write to them registered saying that you have made an offer to make full reparations for the outsanding amount and if they don't accept you will deem the agreement paid in full.