How exactly does a drugstore robbery work?

I am a little bit confused about the robbery of a drug store in GTA Online. So I have some questions:

After the robbery, I sometimes get a 2-star wanted level, and other times a 3-star wanted level. Why does this happen?
Is it harder to lose the cops after a robbery? I have a lot of problems after a robbery to lose them.
Sometimes, when I drive by a drug store which I already robbed, I automatically get a wanted star. Why?
Do you have some other usefull hints for me?

Solution 1:

When you kill the cashier, you'll get a 3* wanted level. If you just threaten him, you'll get only 2*
No it isn't! It's always the same strength of the police at the different levels.
This is because they know your face or you drive with a stolen car. If you drive with a stolen car, this "random" star can appear everywhere.

Tips

Don't shoot the cashier. Just shoot behind him a try to don't shoot him. So he will carry the cash even faster.
Before you enter a convenience store, make an escape plan. Look for LS-Custom's, winding streets or something. If the police is looking for you (if the wanted stars are blinkng), drive in a LS-Custom and go out again (you don't need to change the color), or drive into a winding street and wait. Edit: Just found out that your garage / appartment is also a good place to hide. If you have 3*, drive fast into a train tunnel while they're looking for you (blinking *). So the chopper doesn't see you anymore and if you were fast enough, the police cars also don't find you.
Here I have marked (red rectangle) my two favourite convenience store's:
They're located ad the bottom of the Sandy Shore desert. When you robbed the store, just drive into the LS-Custom (green rectangle). So you loose the police quickly. And Because the LS-Custom is as close to the store's, the police won't be there and won't see you before you're in the building.
Change the car to make it harder for the police to find you.
Before a robbery, buy a mask on the LS-Beach. Then put on the mask and do the robbery. Now, if you put off the mask, you'll loose directly 1* if the police don't see you! And if you enter the store later without a mask, he won't recognize you.

I hope I could help you a bit. And just google for other GTA (online) tips, so you may find some other great tips.

Credit: Map Shortcut made by GTA 5 Map