<human>: So recently I went to a coffee shop by my work to pick up some iced coffee and saw the most beautiful cashier. I bought my coffee as usual and walked out but I could've swore I knew this girl from somewhere. Then it came to my realization that she was my ex girlfriend's (4 years ago) best friend. 

I went in there the day after and told her I felt like I knew her from somewhere and she said "really?" So I asked her name etc and I said "do you know *ex girlfriend*?" Of course I knew the answer to this but I didn't wanna come off as creepy and she said "yes! She was my best friend back in highschool." So we got to talking and she said that she hasn't heard from her in a while and she's kinda hard to get in contact with. I bought coffee and left. 

Fast forward a week to today and I went in there to buy coffee, and talk to her of course. So we had some small talk, which is common for a cashier and a customer to do since I'm a cashier at my work as well, but she seemed genuinely into the conversation. I got the coffee and left. 

So the reason I gave you all this background information is because I need some advice on my approach. Is it sleazy to hit on your ex girlfriend's best friend? I plan on going back there in a few days and talk as usual but leave a tip in the tip jar with my number attached to it so I leave the decision in her hands instead of straight up asking her while she's behind a counter feeling "trapped." Good idea or no?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Want to give cashier at coffee shop, who happens to be my ex girlfriend's best friend, my number. What do?