Good thing is that you assume attraction. Just to make a distinction though, if she gives you that "fuck me" look it is good to assume attraction and perceive it that way (which you already do), however in reality it may not be "fuck me" look at all, it may just be the way she looks.
So you correctly assume attraction but at this time you don't really know whether she is truly interested or not. If it was outside of work it doesn't matter, but she as a customer which might be a problem, you simply can't start asking your customers out as you might lose your job.
So good thing is to listen for some clues and hints. It could be many things, for example, you mention that you are not working tomorrow and two minutes later she says: "I am going to XYZ tomorrow afternoon" while she is looking in your eyes. That's sort of an open invitation, especially if she is quite specific about the place and time (she is telling you directly where is she gonna be and what time). So you say something like: "Oh really? That's really interesting because I was just planning to go to the same place. Maybe I'll see you there, I'm thinking 3-4...??" Hopefully she'll give you an answer, or better: she will be there...
Keep it simple for yourself: if she doesn't go out with you she is not interested, no matter how sweet she is or what looks she gives you. And if she goes for a date she is interested, no matter how bitchy or disinterested she appears... Don't get entangled into "if I did this and that maybe she would have gone for date, or if I say this she may change her mind". It's a waste of time, she simply either goes or not...
Look for good vibes. Look for when she is e.g. coming back specifically to you and ask more (silly) questions about the product/service, look for when she's talking more about herself or perhaps even asking for more personal questions about you. She's basically taking her time and talking more than necessary to show you an interest.
You can also ask for a phone number, something like: "Too bad you are a customer, otherwise I would ask you for a phone number and take you for a coffee". If she is not interested she will just giggle, and if she is interested she will happily give you her phone number.
Also, you don't really need to be flirty. I wouldn't spend much time on flirting, it's a waste of time and effort. Focus more on chatting with the girl, good conversations, deep diving, being a genuine man (see articles on main GC site). Those are much more important. As long as you are talking here and there, and there are good vibes, you should be good. Keep it always light and fun as you already wrote...
