Something in my bicycle pannier fell out one Saturday. A passer-by called out, 'Sir, you dropped something!' (Me: in my 40s, passer-by: male, maybe 30) I don't really use sir a lot, but on this occasion, I remember being pleased. So it could be effective.
(I think I am en 'excuse me' or 'hello' person, like Divisortheory.)
As a regular customer of the store, you can simply use the usual: Hello! Good morning, Good afternoon. Etc. Followed by the usual pleasantries: How are you? Lovely weather. Raining like crazy. Etc.
At some point, if you want to know his name, you can just say, "You know, I've been coming here long enough that I should introduce myself: I'm Abigail." Then, if he has any manners, he'll reply with his name.