I took my 2000 (OBD II) nissan sentra to a repair shop and I had to get my catalytic converter replaced.?

When my repair was done, I drove off and about 5 minutes of driving off the lot, my "check engine" light showed up again. I took the car back into the shop immediately and they discovered that both of my O2 sensors were bad. They now want to charge me for diagnosing the problem and told me that both my o2 sensors need to be replaced. I feel that this shop might have caused the problem accidently or even purposely. I know to access the cat converter requires some messing around with the o2 sensors so could these new car trouble codes be caused by their own sloppy work? This is a AAMCO franchise I am dealing with. I supplied my own part (they wanted double for the same EXACT part) and now they are telling me "if i went with them, I would had warranty on my labor." Any advice?