Shelving should be low enough for staff to adequately monitor the store from a central location. Elevate the floor behind the till and install mirrors. Move cash registers from the back of the store to the front and, if possible, in a position where tellers can see and be seen from the street. Video surveillance is a must. Even a fake camera will deter theft.
Employees steal too.
While it is true that employees steal far less frequently than strangers, when employee's do steal, they tend to steal much more than the stranger. Be aware of the opportunities for employee theft and try to reduce them. This will eliminate most employee bogus refund frauds. Keep an eye on your store's immediate perimeter. Remove debris such as stones or bricks which could be used to break a window or vandalize.