I’d send a copy of the cancelled check to the collection agency with the note indicating the dates you informed the company of said payment. Follow up with a call.
Then I’d call the company and ask to speak to the manager (is it a dr’s office? Ask for the office manager/accounts supervisor) and explain the situation and follow it up with a letter outlining the issue, the dates and who you spoke to at the collections agency, etc. I’d ask them to read any notes on your file to you regarding payment as well.
If you were referred by a doctor of yours I’d send them a heads up letter and cc the brace company and tell them that your service has been severely lacking.
Basically, I’d throw everyone in the loop and make a big stink.
Then I’d start calling the same people every day until they send you proof that your account has been corrected.
BE THE SQUEAKY WHEEL!
Take names and document, document, document.
Seems like overkill, but I CAN’T stand people who can’t do their job. There is no excuse for the hassle you’ve been through, so move to the offensive and put them on defense.