What Are The Laws On Salary and Overtime?

[MTG#10]

At my place of employment, I am paid a set salary for a delivery route I run. Ive been there almost six years now. During the six years, if I busted my ass and finished early, I got to go home. If my truck broke down, or something else slowed me down and I finished late, tough shit - no extra pay. They pay me based on what they say is supposed to be a 50 hour/week route. But to get out of paying me overtime, they put on my check that I work 40 hours then the extra money for the supposed 10 extra hours is labeled "supplement pay" and say I am payed salary.

Lately because of the economy and gas prices, they are becoming major tight-asses and expect us to stay and do other things if we finish early. Basically they want to have their cake and eat it too. We're hourly if we finish early, but if we finish on time or late we're salary. Does anyone know if this is legal? Im tempted the next time I finish early to tell them if they want me to stay to complete my 10 hour day, they need to start paying me overtime pay.