Creditor harrassment vs. willingness to pay

The facts:
a. My wife rented a brand new trumpet (this past spring) on behalf of my school-age son to utilize in a brass band at school.

b. We were to make monthly "rental" payments.

c. My son gave up on the instrument within 2-3 weeks. He "stored" the instrument (in the case like brand new) up in his closet - out of sight, out of mind.

d. We were never sent a montly bill of any kind. EVER.

e. Months later, Collectors starting harrassing me at work on behalf of the music store.

f. My wife then took the instrument back to the store to return it, and to pay in full any outstanding rental balance that had accrued.

g. The music store refused to accept the return of the instrument; apparently(?) due to collections proceeding had already begun.

h. My wife then requested, and received written confirmation from the music store (at the time of attempted return) that they refused to accept the return of the instrument.

i. I am now being harrassed at work constantly. I'm being bombarded with assertions that "I don't pay my bills", "We're going to sue you", "We're going to humiliate you at your place of business and serve you papers there", "We're going to have your employer garnish 25% of your check", etc. I have instructed the Collections Agency (each time they call) that I am NOT allowed to be contacted at my place of employment (my wife is un-employed; thus the reason they want to garnish MY wages). They refuse to stop calling anyhow.

j. The Collection Agency further slanders my wife by stating that "she fraudulently and malisciously stole the trumpet" because she "paid" for it with a "bad" credit card. When asked for which card they were referring to, they described my ATM card. Since the card couldn't cover the ENTIRE cost of the trumpet when she left the store (I believe she was only making first months payment); the Collectors now inform me that LEGALLY, my wife "stole" the trumpet - thus the music store is legally entitled to the full amount of the trumpet.

k. They now state that they will sue me, and that I must pay the entire amount of the trumpet (we only intended to rent monthly; not buy), plus pay the collections costs, and legal fees.

I'm considering taking the trumpet, packaging it at the post office; insuring it (contents included), adding a check for the entire rental from when we took it out of their store until the day we attempted to return it, and have it mailed (return receipt requested) to the music store.

These are the "straight facts". What are my rights not only as a consumer, but as a individual (to be protected from harrassment)?

Thanks in advance.