When two hot women come into your store at 9pm, you have an odds on chance of getting flirted with, but you do have to do some work to make it happen.
Among your options are things like making witty comments about their options, the weather, any other topic that you all might share. You could also compliment them on how they look. As a rule, if you want to flirt, it's important to give a really clear opening.
You'll note that among your options I have not included smiling in an awkwardly skeevy way while waiting for one of them to ask if you happen to have a *wink wink* back room that you could show them. If you do this, they will find you awkward and skeevy and they will probably grab the alcohol you are handing them and book to the front of the store as fast as they can.
At the counter, if you're trying to flirt with them, you probably won't find it a good opening move to say, "Back already, huh?" to someone who has never been in your store. It turns out that many women in the world color their hair and wear red, and they are not interchangeable.
When one of the women has not brought her ID, but tells you she's 30 and has a 5 year old kid, regardless of whether she buys into the youth=beauty myth, you will not blow her hair back by telling her you had mistaken her for 27, 28 at the oldest.
Finally, in general, if the skeevy men working in a store outnumber the women in the store, they are unlikely to stick around to flirt with you once they have their booze and can leave. You can increase your odds of having beautiful women hang out and flirt by chilling out and getting just a tad smoother with your approach.
Okay, a lot smoother.