It's the owner's store! Replacing the manager won't stop the thefts, unless it involves somebody turning their back while the theft is occuring. The manager should only need to propose a solution, the owners should pick up the tab (does the lost profits out weigh the cost to control it?).
Packing an alarm inside the case most likely won't be very effective, and costly.
RFID might be a good investment, but depends on the sales volume and actual loses. Could be costly to setup.
A low tech and cheap solution, hang out at the store, and watch the customers, maybe you can catch one in the act. I'd think your theives are most likely kids, teenagers. Thefts, shortly after school hours. Mostly groups of two or more. Most adults could download even new releases off the internet and burn a copy in a day or two, and less likely to risk jail for shoplifting...
Mirrors have been a good deterent, and not very expensive. Cameras, whether functional or not will help. Unfortunately, society is a little to forgiving of kids commiting crimes. Parents are too limited in how they can legal discipline their children. Fear of getting the hell beaten out of me, kept me out of trouble, for the most part...