How do I ask cute cashier girl out or know if she is avaliable ?

Anonymous
Hey I need your help. There is a girl that works as a cashier and she seems to be a bit shy. I have seen her (and she has seen me) twice and the second time I was there I gave her a note that said: "add me on Facebook, I want to get to know you" and then my name. I didn't want to give her my phone number because why would she call or text someone she doesn't know ? So on Facebook she can get to know me what I like etc. But she didn't add me and it gets me frustrated. My question is should I go back there and ask what time she is finishing work or what should I do ?
How do I ask cute cashier girl out or know if she is avaliable ?
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