My opinion is that if you did nothing on your side to engineer the non-charge, you really don't have an obligation to try to correct it.
I can think of 3 instances where I recieved goods/service where I didn't end up paying for them: one was a debit card POS purchase (well before they offered MasterCard/Visa-branded debit cards) for a small amount, one was a Visa debit card purchase for a small amount, and one was a $250 hard drive charged to a regular Visa (I had received the wrong item, returned it for a replacement, and received credit for the return but never charged for the replacement).
But, if the merchant figures it out and charges you later on, you of course have an obligation to not dispute the charge and pay for it.
During the same period of time, I can think of three chargebacks I've had to initiate for incorrect merchant charges: a restaurant that charged me twice, a health club that continued to charge me monthly even after I cancelled in writing and got a written confirmation, and an item I mail-ordered but never received. Of course, I expected the merchant and/or the bank to make me whole for these mistakes that were discovered by me after the fact, and it's only fair that I do the same if the shoe is on the other foot.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">A couple of weeks ago I received a letter from a car rental company. They told me that they checked their books and that they forgot to bill me one rental more than a year ago (15 months!!!). As I rented a lot back then I didn't even notice.
Now they "asked" me to give them the permission to charge me. Legally I don't have to pay because it's so long ago.</font>
I'm not so sure about this. In many states, the statute of limitations for civil claims such as this is 2 years. I think it's great that the merchant contacted you, but I'm not sure that they had do (perhaps they have more restrictive terms in their credit card merchant agreement). Had they not been able to charge the credit card, they most probably would have been able to bill and collect the amount directly (whether it would be worth their while is another matter).