In this day and age, why would a near total stranger give you her number? I agree with Punkey's response as to the way you should have handled things.
The girl's response to you seems very likely a blowoff. You did not ask for a relationship, but simply for her number. She immediately went to the "not ready for a relationship" excuse, so yeah, I think you should back off and forget about her.
As for going to the store, if you need to buy something at that store then go to that store. If you make a habit of staying away from stores where girls who won't give you their number upon first meeting you work, then pretty soon you are going to have to travel great distances to find someplace to shop.