Remy, there's a big red flag in your post- the payment was to an MD. I'd check to see if the check was cashed. Doctors get office help wherever they can-usually the "temp of the month". I just got through paying a $30 copay, supposedly that I didn't pay in Jan 2023. When I checked my credit card, I did pay it. I had to get a refund. This is not the first time this happened with this doctor, but three of my docs. And if your doctors are anything like mine, you had to pay the copay before you can see them. It's hard to miss a payment.
The US Post Office is lousy delivering packages, but they have perfected letter delivery.