I never actually bought a full cat-back exhaust. Sounds like your mechanic is either not a muffler/exhaust guy or is lazy. I went with the universal N1 muffler and just had the rest of my piping made. It's not a big deal to get a universal muffler and just integrate it into your exhaust, especially if you're not looking for performance gains. All they have to do is hack off your muffler, weld new hangers onto your new muffler and weld it onto the existing piping on your car. They might have to expand the piping on the muffler so it will slide over your existing piping, but that's common. Just spend the ~$150 on the muffler and get them to install it. Very simple process.