Bucky's Screwed Over My Mustang?

So I took my 1995 Mustang to Buckys to fix my bad catalytic converter so I wouldn't die from carbon monoxide. The person in charge made me a deal for $375 to fix my catalytic converter and he would also put some flowmaster 40s on it next time i came in. 2 weeks later i took my mustang in and also asked for an oil change which would be an extra $15. So he told me he'd do everything for just $375 with no paper work and i agreed. A few hours later i picked up my mustang. The guy told me he put some other muffler on it besides the 40s, some brand that starts with a z. I didn't really care since it sounded better than it did before. I've been driving around for around a week and I've noticed it sounds like they did a crappy job on my car. My catalytic converter and muffler sound like they're loose and or scraping something. Now that i actually looked online for catalytic converters i see that they're only about $100 for a good one. So I'm not sure if it was worth paying $375 when i payed about $200 for labor. I just plan on going to Buckys on a busy day and complain about my car to get it fixed for free or something. But im not sure if they will say something like i dont have any papers or proof of my car being there. Opinions and help is much appreicated! What should i do??