I wouldn't read too much into it...
Most shop managers want their staff to be extra friendly and nice, so the customer is satisfied with the service and more likely to return (who would you rather be served by, someone polite and friendly or cold and rude?) . I'm not saying she doesn't like you, but I think it's unlikely.
Having said that, there's no harm in chatting to her and building your confidence in talking to girls (it really helped with me anyway). Just don't read in to much of it at this stage... just take it for what it is, a cute checkout girl who's easy to talk to.