1. Make Sure its mandrel bent when u ask for a quote...
2. Check to see if they screwed u over by not using mandrel bends BEFORE u pay..
3. Inspect the car while its up on the hoist.. a good reputable workshop should let you do this
4. Don't go to Non-Exhaust places to do.. (they WILL mess up the job bad... e.g. Wheel Places offering exhaust fitting...

Note that custom exhaust don't fetch as high a resale value as a aftermarket JDM one.. if the quote for the catback is in the region of 600++ i HIGHLY recommend getting a JDM one..

Cheers
Hope This Helps