$1,000 limit for six-month rule on parts and accessories is cumulative

A customer bought a truck from us in March. In April, the customer brought the truck back for installation of certain parts and accessories. The price of these parts and accessories, including installation, was $800. The customer told us no other parts and accessories had previously been installed on the truck. We told the customer they did not owe any tax in connection with these parts and accessories because the price was below $1,000. In May, the same customer asked us how much it would cost to install an additional part on the truck. The customer told us that no parts and accessories had been installed on the truck since our April installation. We determined the price of the additional part and its installation would be $400. Because the customer has now asked us to install parts and accessories, that have a combined price of $1,200, we think the customer would owe tax on $200 -- the amount over the $1,000 threshold. Are we correct?