I've been dealing with these people since March 20...

I've been dealing with these people since March 2018. They sent me a bill, which I paid for with the wrong credit card (mastercard). I immediately called them and asked if I could pay with a different card instead (HSA). They said I could, but I would have to wait to be refunded from my mastercard until the manager was in on Monday. They called me a few weeks later to say everything was taken care of, but it wasn't, because they refunded my HSA, not the mastercard.
We go back and forth for weeks to get the issue resolved, but they never refunded my mastercard.

The manager never calls me to resolve the issue, she makes the people beneath her do it. So now (8 months later) I get a letter telling me they've sent my bill to collections. How? This bill as been paid for 8 months? I'm waiting for someone to return my call, and filing a complaint with the BBB in the mean time.