If you haven't had to work a crappy part-time job, or you've completely forgotten what yours was like (likely you've supressed the memories), consider yourself lucky. The world of customer service is tough, especially since the managers have all the power and you take all the crap from the bad customers. The question here is, are you one of those customers that makes the employees wish they could justify strangling people? Or are you one that gets what you need while remaining friendly and civil?
Find out whether all the cashiers run and hide when they see you coming, or if they draw straws to see who gets to serve you... and hopefully you'll learn just how much being friendly can earn you--respect, courteous service, efficient problem-solving, and possibly a few freebies or extra help above and beyond what is expected. Who knew being nice could be so helpful?
PS: Please be nice... this is my first test, and I'll update it as any (constructive) critisism comes in.