It all comes down to the invioces and whats been paid and if the OP initial post is genuine (I have no reason to believe other wise).
As I understand it the OP had seperate invioces, one for the vehicle (A/P) and one for the Accessories, both invioces where paid in good faith that this was the full and final payment by the OP on the day the vehicle was collected, PIN entered, all paperwork checked and signed, vehicle keys handed over and the OP drives away.
If this is the case then any error on the invioce is down to the vendor as the OP has paid all monies due for the goods in good faith, the vendor realising that they have undercharged have sent another invioce for the amount they have undercharged hoping the the buyer will pay, it is up to the buyer if they pay or not.
However had the vendor realised the accounting error on the invioce before the buyer had paid then they it would be a different matter.