Look at your situation the best thing is several containers of mace. I recommend heat brand products, they have ones that look like simple flashlights, and test cartridges so you can practice firing them.
Back when I was stupid, I was robbed at gunpoint in LA, and I fired my pepperspray keyring without thinking. The suspect immediately hit the ground screaming in the most pain I've ever heard, at which point I kicked his firearm, emptied it and called the police.
It was probably really dumb, but pepper spray works really, really well.
A while back a major network did a test of the most effective self defense weapons, tasers, pepper spray, and even a dummy firearm ringed with paint.
Of all the weapons the only one that protected the victim in the fake attack was the pepper spray. The attacker safely disarmed the victim everytime especially with the gun, because people hesitate to use it, and it was always recognizable.
If someone robs your work odds are they are going to do it with a gun, and be 3 feet away. I don't see you doing very good with a knife, and not getting shot, it takes a long time for somebody with a knife wound to go into shock, or bleed out, and you'll only make them angry.
With mace (heat brand) on your keychain it looks like a little flashlight on the ring of your keys, which you can simply say you need the your keys to open the register...
Also having several containers of pepper spray (they even make a pepper spray ring, but I don't know if it shoots far enough), + a bat is a great combo, because after spraying, you can attack with the bat to get the weapon out of his hands.
Honestly though if you don't think you are going to die, I'd just give them the money, it's not your store...
In 1998 in California a bank robber with a ak47 was foiled when a teller sprayed him with pepper spray. He ended up on the ground, and fired some shots in the air because his finger was on the trigger, but immediately dropped the weapon and surrendered.
Pepper spray is NASTY stuff, it won't kill, but the right spray will totally stop someone, they won't even be able to think. I won't encourage you to spray yourself, but I sprayed some outside, near me, and it never touched me but I was in horrible blinding pain, and the smell made it so I couldn't breath. I can't imagine actually getting it in the face, it has to be horrible, just being 3 feet from where I sprayed it was enough that I'll never test spray a real canister again.