Well, all I can suggest is a few things. I am not familar with what your local law says about any of these things, so I will make several suggestions.
Park, if not close, within sight of your store. That way you can see if anyone is hanging around your car and can take measures to protect yourself. Like calling the police, if you deem it necessary.
If you are closing always ensure there is at least one other person closing with you. Closing is the worst shift for such things as crooks will know when you are leaving. Probably best not to buy a game to take home at this time, either. You don't want to have obvious valuables on you at this time.
If you can, buy a gun and get a concealed carry permit. This assumes you would be able to take it into your store, which you should check into first, and that your country even allows this. I know some people here will say that carrying a gun is a bad idea, but ignore them. If you can, this would probably be a good idea.
Failing the gun, due to laws, or unwillingness, pepper spray may be a valid alternative. It's nonlethal and generally effective enough to give you a chance to escape.
Finally, if nothing else, carry small amounts of money, avoid carrying credit or debit cards (at least in your wallet) and just give them your wallet and any money in it. Can also save you, if someone mugs you in to store during business hours. Sounds like a cowardly way out, but it may save your life. Immediately following any such event, call the police. Only problem I can see with this is that it could make your or your store employees a target for future muggings, being an easy mark.
Beyond that. Good luck. Hopefully, you never need to worry about any such things.