We run a local appliance and refrigeration repairs business. We charge customers a standard call out fee, but there's one time I forgot to mention this call out fee to a customer. So I sent the technician there to fix a stove. We inspect and diagnose the problems for the customer. We told the customer, but he said he wouldn't be able to make the decision as he needed to wait for his dad to make it. He paid the call out fee and we hand them a receipt and he signed the receipt too. Then later that night, his dad called me and tells me because we didn't mention about this call out fee, he wants to get all the money refunded and I told him we can't and he said then at least half of the money refund. He said because I didn't mention about this call out fee so its our company's fault and he wants his money back. I told him our technician was there to help you diagnose the problem and you have to pay us. He said if I don't pay him he is going to ask a lawyer to sue me under Australian Consumer Law. I'm just really worry about it. I need help.