the store interviews will be pretty straightforward and simple if u have made it already through the goup interview n all that (i assume u have made it through the group one to have gotten an in store interview).
um, basically dont stress there will be a few simple questions, "tell me about yourself" and "how have u dealt with a confrontation in the past" or "what do u do if someone has a problem with you" etc etc and then theyll just confirm your availability n stuff. try and be as open with ur availabilty as possiblle.
my interview was pretty easy cos all i had to do was tell her what uni course i got into, smile a few times and she spent the rest of the interview going OMG OMG OMG OMG at the uni course i got into. (i had my interview the day after offers came out)
just smile and relax.... its a supermarket, not brain surgery so nothin to really worry about. (easier said than done, i know)
let us know how u went!
goodluck