You can convey all kinds of feelings with your voice. Impatience, disbelief, even anger. It's almost impossible to rid the customer of the image these attitudes conveyed. When you interact one on one with people, they have a lot to judge you by. Facial expression, body language are two very important factors.
Appearance also is important.
There are some jobs where casual dress is part of the scene. This is fine in those instances as long as you are clean and your clothes are well kept and not worn. There will be times when you are trying to provide customer service, that a problem might arise on the job you are handling. Maybe a package was delayed, or a promised completion date not met for one reason or the other. It's very easy to become defensive when these situations occur.