Re: She remembered my birthday
Remembering a birthday is a big IOI, and given the fact she's a customer that's just fleeting in and out, it's an even bigger deal. She would remember you and she would remember the birthday because she finds you an interesting person. You say you're confident, and it doesn't sound like you had any issues with you conversations, so she might have found some aspect of you attracting based on your short interactions.
If she comes back, do about two minutes of light, fun banter, and then say "Oh no! I would really love to get to know you more outside of being a customer, but I have to get back to work. Do you like coffee?" and then recommend a time and place to meet up at a cafe to chat.
"There is no better than adversity. Every defeat, every heartbreak, every loss, contains its own seed, its own lesson on how to improve your performance the next time." - Malcolm X