IMHO, unless you have some way of absolutely preventing an employee from ever having access to the cash drawer, for instance, you're never going to have a system where a determined dishonest person cannot steal from you. I doesn't matter what POS system you're using or how you have it set up. If someone is that determined, they're going to rip you off whether it's through discounts or pocketing cash - or, in your case, going home with pockets full of sunglasses.
I think the best anyone can hope for is a system where you can detect that kind of activity easily enough that it doesn't get by you once it's been done.