Send them a letter detailing what you outlined above, indicating what the charge should have been, and with a redacted bank statement or website printout showing that both checks were paid, and for how much. (By "redacted" I mean white-out the accountant number and any transactions other than the two in question.) Ask for a refund (specify the amount) within a reasonable time (e.g. 10 business days).If they don't refund, send them a follow-up letter with a copy of the earlier letter and bank statement; in the follow-up, state that if they don't refund the money within a further 10 days, you will be forced to file a small claims suit for it.If they still don't refund, file the small claims case.