I've got a rental car with Thrifty and were given the hard sell of prepay tolls being the only option to use NZ toll roads. I politely declined as it was going to cost more than any use of the toll road for our short trip.

Thirty's own website mention it's optional, but make no comment on pre/post pay, other than their own add on, or fee based service (at least $25+toll).

Hertz (Thrifty parent company) certainly suggest you can pre/post pay your own way. 

NZTA's website mention users should make sure they use the rental cars preferred method for payment. 

Interested to hear if anyone have used tolls, pre/post payment, and not been chased by the rental firm later on.