Bowdoin Square Exxon Ripped off for going green Boston Massachusetts

Early this summer I brought my boyfreinds pick-up truck to this service station for an oil change. He requested that I get synthetic oil put in, it's better for the vehicle and the enviroment.
I've had my own vehicle serviced at the station several times prior and I've never had a complaint. When I dropped off the pick up truck I asked how much it would cost for the synthetic mobil one oil change. I was informed that it would be "a little bit more" than the standard oil change, which they charge forty dollars for.
When I picked up the truck it was approximately six pm and all the mechanics had gone home, the only employee working was the guy who pumps the gas. He handed me a bill for one hundred and fourty dollars...for an oil change. I laughed and said there must be a mistake. Of course there was nothing he could do so I paid it, in cash, and said I would be calling first thing Monday morning.
When I called to inquire about the steep price they assured me that this was the average price for an oil change with synthetic oil. This is more than triple the price of a standard oil change. The slip was not itemized and when I requested one they said they would send one in the mail, never did. When I called around to nearby service stations I got quotes of sixty to seventy dollars for the same service. I've tried contacting the station several times since then and they do not take my calls. Messages will be taken and I never receive a call, or they simply don't answer.
I've filed a complaint with the headquarters of Exxon Mobil, who agreed that I was ripped off, but nothing came of it.

Young innocent female taken advantage of.
Lowell, Massachusetts
U.S.A.