hi caz, you dont have anything to worry about shop interviews are usually really easy. i worked in argos for a few months last year, and my boyfriends been working there for i think just over a year now - he loves it to bits lol.
When i went for my interview i remember they made me write out some numbers or something like that to see if i was capable with numbers.. but i dont think they do this anymore but if they do dont worry its not anything hard!
good luck, let us know how you get on xx