2001 Cadillac Deville Car won't start. Back to mechanic

Electrical problem
2001 Cadillac Deville Automatic

Hi. I have a 2001 c. Deville. The problem at first was that the car would not start meaning I put the key in the ignition and would not crank (no sound) but my radio, lights, windows etc worked. I started the car an hour later it started. Shut it off and waited another half hour and car would not start. I called AAA and they towed it to repair shop and once again the car started. Repair shop said it was the starter. Ok fine. I got the car back within 2 days. I got it and it started up. Right after that I went to fill up my gas tank and the car started. Then I proceeded to the supermarket down the road and went grocery shopping for an hour. Finished shopping and the car would NOT start! I called the repair shop who was just down the road they towed it back. This is just a matter of two hours from taking my car from the repair shop. So this morning I called and all they could say was it was a defective starter. What is the chance of it being a defective starter? And what else should these people be looking at because honestly I know its going to act up and not start up again because the car is doing the same exact thing from the first day when it wouldn't crank. And also when it was towed back to the repair shop for the second time, miracously the car STARTED! Please help. Thanks