Need some advice for a collection agency

I am furious at the moment, and need some advice from you all before I send the letter.

In June, I was fitted for a knee brace. The company billed my insurance, and then billed me the remainder — $374.06. I paid the bill and the check cleared July 9.

In August, I received a letter from them, stating that the bill was past due. I called, explained the situation, that I had the canceled check. They promised to straighten it out and get back to me.

About three weeks later, I received a letter, stating that the bill was past due and if I didn’t pay it in 10 days, they would send me to collections. I called, explained the situation, that I had the canceled check. They promised to straighten it out and get back to me.

Earlier this month, I received a letter, stating that the bill was past due and if I didn’t pay it in 10 days, they would send me to collections. I called, explained the situation, that I had the canceled check. This time, they asked me to fax them a copy of the canceled check, which I did. In the meantime, they promised to straighten it out and get back to me.

Today, I received a letter from a collection agency, stating that the bill was past due and had been sent to a collections agency.

I plan on sending them a letter with signature confirmation, explaining the situation and including a copy of the check. Is there anything else I need to say? Any way I can make sure this does not go on my credit rating? Anything I can do to make this stop???