Car lot wants more for car paid in full

We just bought a 01 Ford F250 from a used car dealership. We paid cash for the truck and received a reciept that said Paid in Full. When the dealer mailed us the license plates, it came with a letter stating: There was a mistake made on the amount that you were charged for you licensing fee due to a computer error, you were charged $35, the cost was $147. The difference of $139 is the balance owed. You can mail your payment withing 30 days of the date of this letter....My first question is can they ask for more money when we paid in full? Second, the difference between $147-139 is only $8, not $35. Is this a plot for them to collect more money and if so how do we take this further? It is not that we have an issue paying the correct amount owed if this is legal, but it seems very strange and it is the principal of the issue if they are trying this on other customer's. Any suggestions or help would be greatly appreciated.

You need not pay. You could have decided not to proceed if they had been correct originally.

You may also report them to your state attorney general's office.