being billed for an item that was already settled

by kathy
(colorado springs, co)

Last year I went to Lincare to get a nebulizer for my son. They were to bill the insurance company. But the person that filled out the paperwork checked "Own" instead of "rent" The insurance company wouldn't cover it. So a year later they tried to recover 12 months of rent payments ($11.00 month) Turns out they just changed the paperwork back to rent when they discovered their mistake but never told me about it. I refused to pay but offered to pay the two months of rent and they agreed and sent me a bill marked "final" I offered to return the machine but they said they couldn't take it back. Fine. I paid the remaining on the bill immediately. Now ANOTHER year has passed and they are once again billing me for over $100 for rent for the past year--even though I have the bill showing the same account as FINAL BILL. What are my rights here, they still won't take the machine back but say I owe them the rent. I've googled them and they have lots of billing problems and even medicare fraud in their history in some other states. I live in colorado.
thanks