Most doors that lead from the sales floor to the stockroom have an exit sign above the door. An EXIT sign is a good indicator that the door is in the "path of egress" and cannot be locked.

The National Fire Protection Agency (NFPA) and the International Building Code (IBC) both state that these doors must allow someone to exit, during an emergency without any special knowledge. For this reason a push button lever lock on the door that leads to the stockroom is a violation of life safety codes. Push button lever locks are great for limiting access to a manager's office, a closet or access through a door that is not in the path of exit signs. 

You can put an alarm on emergency exit doors, which will alert staff that someone has used the door in an unauthorized manner and an exit alarm does not violate life safety code.