Ask your shop to measure the parasitic current drain. It should be less than 50 mA. If more than that there is likely a problem with something not turning off all the way when the car isn’t being used. If you don’t drive the car enough, that will eventually drian the battery, but if your report is accurate above then that should be enough to keep the battery charged. No harm however to top the battery off with a battery charger once in a while.
Another idea, maybe the battery isn’t the problem. When you say it had a dead battery, how did you know it was dead? B/c the car wouldn’t crank when you turned the key to start? Or something else?