Mr Fulton rang a local Vehicle Testing New Zealand office to see if there had been some mistake at the Post Shop.
A woman confirmed the same charges as the post shop had but was unable to explain the difference.
Confused, Mr Fulton turned to the Bay of Plenty Times to see if the newspaper could get a satisfactory explanation.
If such a charging mistake was happening to him, it could be happening to others, he said.
However, other people were unlikely to notice the overcharge because not many people have two identical vehicles, he said.
The Bay of Plenty Times contacted the New Zealand Transport Agency (NZTA) to ask whether Mr Fulton's situation was a glitch in the system or an example of a systemic failure.
NZTA spokesman Andy Knackstedt replied that one of Mr Fulton's vehicles had been incorrectly registered as a type-two vehicle since 2002.
The incorrect classification had not affected the charges until a new charging scheme was introduced in August, Mr Knackstedt said.
"Prior to 1 August 2012, the RUC rate per 1000km was exactly the same for RUC vehicle type 1 and 2 at the weight of 2 tonne, which both Jim's vehicles are - ie no 'extra' RUC was paid due to the error by the licensing agent.
"Since 1 August 2012, a RUC type 2 would pay $52 per 1000km plus a transaction fee, while a RUC type 1 of the same weight would pay $48 per 1000km plus transaction fee."
The incorrectly registered vehicle had now been correctly recorded as a type 1 vehicle and Mr Fulton would be able to purchase RUC' at the same rate for both vehicles, Mr Knackstedt said.
The NZTA's RUC team had encountered "very few" situations similar to Mr Fulton's, he said.
