mercury tracer? I take it your talking US dollars not aussie peso's. IMO you are getting seriously ripped. As your parents made you buy it, they really should pay for the repairs
NAPA really should give you a credit for the shitty fuel pump they supplied you (which they don't stand by and didn't fix your problem) and the you'd only be up for another 100 not 270. I would get pretty angry with them if they didn't do that.
You can't be expected to pay for their incompetance, they should've been able to work out the reg was the prob before frying another fuel pump. It really sounds to me that they're only guessing and I would be surprised if a new fuel pump would fix it. It still drives doesn't it?
I bet they say the injectors need doing next.
p.s. dont go in there pissed off, try to work it out first.
Try and find a mechanic you trust, ask friends and associates for recommendations. And let it known that you're at this new mechanics because youv'e been ripped off and aren't going back to Napa.