Is there a physical address on the invoice? Write a paper check (or get a moneyorder / cashiers check), attach a copy of the invoice and a quick letter explain

Is there a physical address on the invoice? Write a paper check (or get a moneyorder / cashiers check), attach a copy of the invoice and a quick letter explaining that their system won't let you pay any other way, then go to the post office, send the whole thing by certified mail and be done with it.