I’d be surprised if 500 miles per month isn’t enough to keep a new battery fully charged. When it didn’t start, do you mean it wouldn’t crank, that rr rrr rrr sound? Assuming so, there is probably something remaining turned on overnight & draining the battery. Ask the shop to keep the car several days and monitor the phantom drain current on the battery as it sits. It shouldn’t go above 50-100 mA once the car has been turned off and parked for 30 minutes.
If it goes above 100 mA then there is some circuit that causing the drain. With newer cars what can happen is a device fails (it could be the radio or the gps or the alarm system for example) and that confuses the computer system and causes it to remain on even after the car is parked. The drain test above will identify if that’s what is happening.
Also measure exactly how much you are driving. You may think you are doing 500 a month, but really only doing 150, and that could be contributing to the problem. 150 miles a month might not be enough to keep the battery charged on newer cars. In which case the battery tender is the only option, other than buying an older used car.