yeah unfortunately you will always encounter these type of people no matter what job position you are in if you are in a postion where you have to deal with people.. ive learned to just smile and not let it affect me.. company policies are usually "the customer is always right" so there is not much you can do unless they get out of line and that is where you can tell your manager but other than that.. just try not to let it get to you! i know its hard at first.. also remember even though they may be wrong, you dont know what kind of day they have been through and they may be just putting it out on you (as wrong as this sounds) so the best you can do is just learn from it and try not to let it get to ya
good luck!