Going back 20 years or more every one I ever rented was based on size, time and a weight not to exceed X amount.
Weight over X always entailed extra charges.
Recently those maximums were lowered and a per ton charge was added. so it might well depend on location like so many prices on the Internet do.
Shingles are heavy and I can see extra charge for a 3rd layer. My personal opinion is that the amount of work increases in direct proportion to the layers. Strip a few roofs and get back to me with your opinions.
Colbyt