Mail carriers are federal workers who make adequate pay and have good benefits. I am not about to tip them.
Restaurant workers who make an inadequate hourly wage get tipped as long as their service is not ungodly horrible!
However, I AM sick of tipping hairdressers who already charge a very adequate amount to give a haircut. I don't mean those who charge around $20-25, but some who charge $35-40 and more don't need tips on top of it.