At my doctor's office I paid a small copay, as one often does at a doctor's office. I paid in cash and, when asked if I wanted a receipt, said I didn't need one. I normally don't take receipts for cash transactions if I can avoid it, since I don't need to check cash receipts against my bank statements. As you will see, I have learned my lesson.
The doctor's office later called me and then later sent a bill saying I never paid the copay and are asking for it. I obviously have no real proof that I paid the copay, and since the doctor's office isn't careful enough to have recorded I paid the copay I assume they're probably not careful enough to check their cash drawer every night and know whether they have extra money or not.
Is there anything I can do to convince them I paid this copay?
Edit: Please note my question is "is there anything I can do to convince them I paid this copay", not whether I should pay the copay a second time or not. I am willing to pay the copay a second time, but only if I am left with no alternatives.