Repeated Failure to Send Proof of Payment

I have asked twice now for proof of payment to the doctor for my mother's outstanding account. Both times your staff members sent me a Guardrisk letter to say Admed has paid. If I send Guard Risk a letter from myself to say I have paid for something, will Guard Risk accept my word that this is proof of payment? What I have been sent twice is not legally considered proof of payment. A third party, in this case, the bank has to send proof of payment.

Send me proof of payment from your bank. The last time I contacted the doctor's accounts department, they were also struggling to get proof of payment from Admed/Guard Risk. If the money cannot be traced, then it is reasonable to ask for proof of payment from the bank and not just a letter from Guard Risk promising they have paid the money. That is not proof of anything. Please send proof of payment from your bank.