Well..............First off don't be that hasty to blame the shop, you are assuming it was their workmanship that is at fault and you know what "assume" can mean! As has been mentioned. Check the battery, the cables, the size of the cables, whether or not a cable is getting hot to the touch while cranking the engine, Remove the ground stud from the engine, clean the threads and replace the stud (poor ground due to years of corrosion), there is a chance that the solenoid has poor contacts and may need replacement. Lots of things can create the issue you have. Chances are before you got into a pi__ing match with them you could have taken the rig by there for them to verify their workmanship and possibly identify the issue.