Carl's Buick Pontiac GMC Dishonest, no reparations, lied about having my car all fixed, broke down on my way to school 24 hours later. Won't refund my money or refix my car Stuart Florida

After other mechanics were unable to fix my car, I decided to take it to a GM pontiac dealer because my car was a pontiac, and I figured they would be the ones to help me. Little did I know, I was walking into a bottomless money pit. On the phone with Carl's Buick Pontiac GMC dealership, they assured me that they could diagnose it. I brought it in, and was told it was electrical issues after being put on a computer and such. They assured me that the mechanic would find the problem and fix it if I paid 100$ per hour labor for him to search my engine for the problem. He estimated three hours or less, and that was still too expensive for me. I had them do it anyway, and they assured me that it was a melted relay circuit and that they fixed it. Upon picking up my car, I was charged 550$ which was a lot more than my estimate. I didn't care, I was happy to have my car fixed and running again. 

I drove the car home that morning and didn't start it again until I left for school the next day, 24 hours later. All was well for five miles and then the car powered off on me. I was stuck in the middle of traffic, late for my chemistry lab. When I called them, they blew me off completely. Gary, the GM of service, told me that there was nothing he could do because he fixed my car already and that I must have broken something else. I didn't drive the car at all after picking it up except to drive it home, and there's no way something else in the car went bad overnight. He did not diagnose it correctly and I want a refund or work done at no cost- THE RIGHT WORK THIS TIME. Paying a business for their service means you should get what you pay for, and I didn't. 

At my job, if we do the wrong thing we must redo it. For free. Always. Or give them a refund. We get paid to do a job and when we don't do it, there are consequences.

now my car is back at his shop and he wants to charge me 750$ for a new computer. The computer in my car is brand new and shouldn't be replaced again. I cannot keep pouring money into a guessing game of engines. I am a full time student and employee and this company took advantage of me! Now I'm working minimum wage to pay back the money I borrowed to have the work done!