Rental account closed two years ago (and paid in full) just sent me a bill. What are my options or obligations?

My wife and I rented a washer and dryer while in an apartment. When we moved out in early 2009 we paid the balance in full, gave them a forwarding address and they picked up the equipment. We had a check ready for the driver for whatever he said the balance on the account was.

January 1st of this year, just after midnight, I receive an email that said I owed $70 on the account, even though it was closed. I figured it was a mistake probably due to a glitch in their accounting/invoicing software and flipping to the new year.

Two weeks later I received a reminder that I was now late with my payment for $70. I contacted their customer service by email, said I thought there had been a mistake and asked that they clear it up if possible.

4 days later I receive an email from a customer service rep saying the balance was indeed due and attaching an invoice showing the original amount of $70 wasn’t correct, but I did owe $45 for a pickup fee, some small portion of a monthly charge (which was only $32/mo in full) and various other taxes and fees. They said an audit of closed accounts revealed the balance but they had never invoiced me for it. She said they did expect payment for this updated amount.

The invoice has my current phone number, address, and email on it, so they have several ways of getting in touch but the email on the first of the year was their first attempt. None of the amounts add up with the amounts of our monthly bills, even figuring in the pickup fee and the inconsistent amounts don’t make me confident that this new amount owed ($45) is any more legit than the old amount ($70).

I don’t mind paying my bills, and realize the amount isn’t a life changing sum, but the whole thing seems to be a string of errors on their end that has ended up with me somehow owing money. I’d look through my records to prove payment, but I don’t even know what to look for, apparently I was never made aware that this was due.

Can any company I’ve done business with just pop up after I’ve closed my account and ask for additional payment years after we did business? Doesn’t it seem like there should be a time limit on this sort of thing?

Dearest jellies, what are my options to handle this in a civil way? I feel like a letter is most appropriate but I’m not entirely sure what to include. Should I state I won’t be paying the bill? Is there an obligation on their end to notify me of the amount due in a timely manner?

What would you do?