The three gallons of waste oil has to be dealt with according to EPA standards.

The dealer has to pay a company to collect the waste oil. That means a guy with a haz-mat endorsement on his CDL, driving a specialized rig to suck it out of the EPA approved container. The driver is likely making a decent wage and the waste oil company has all it's bills to pay as well, regarding transportation and disposal.

$75 is likely excessive but the shop is paying thousands of dollars to deal with the waste. Someone has to pay for it.