its unlikely youre being scammed, and that being the first thought you jump to makes you seem like a total ass. cars dont tell you whats wrong with them, even engine light codes are only giving symptoms, like going to the doctor with a stomach ache. there are a lot of things that can cause a stomach ache. one of the first things to check in your scenario is the alternator, and if the voltage readings while running werent 14.5ish, that signifies a problem with it, and needs to be replaced. like if you told your doctor you ate spoiled food out of the trash can yesterday, he would reasonably conclude that was why you had a stomach ache. then when you go back a week later and say you still have a stomach ache, he would then do further tests. same thing, he found a problem, believed that to be the cause of your issue, because a bad alternator can absolutely cause the problem you described. now you have the same issue so they had to do more diagnostics to see what else was going on. stop assuming mechanics are scamming you