Mechanic: "Yeah, we can get this non-OEM replacement belt tensioner you want in, we just have to make some modifications"
Customer: "That's great, I'll save $65 on the part, have at it!"
*three days pass*
Mechanic: "OK, the modifications to your engine block cost $90 per hour of machine shop labor and added $360 to your bill, plus we had to order some replacement brackets because they wouldn't fit your non-OEM part, so that's another $268, and we broke a couple bolts off in the engine block so we had to drill those out and we cracked it from the timing cover straight around to the head. But since you wanted non-OEM parts, we're not responsible for the damages."
Customer: "But you said it would be cheaper this way!"
Mechanic: "No, *YOU* said it would be cheaper this way. By the way, I can get you a factory remanufactured long block engine for $2860 installed, or you can have this other one for $2150 installed. Pick one."
*disclaimer...I made this up...never happened to me...never will