Does she have a name tag on?
YES
Approach her. Smile.
Say these words to her:
"Sorry to bother you - do you have the time please?"
When she gives you the time, say:
"Thank you <her name> for your help."
Then carry on with your shopping.
NO
Approach her. Smile.
Say these words to her:
"Sorry - I just wanted to know what the time was - what is your name please?"
When she gives you her name and the time respond with:
"Thank you <her name> for your help."
Then carry on with your shopping.