Tip is an acronym for To Insure Proper service. It's something that you do so that you will be well served the next time.
With someone like the mailman, he's obligated to provide the same service to everyone regardless hence does not need to be tipped and indeed may be in trouble if tipped.
Waiters and waitresses are traditionally independent contractor that either pay a "tip out" fee to work at the establishment or are paid less than minimum wage because they're not really employees, the service that you receive from them has traditionally been optional. We still see this tradition of independent contractors and "tip outs" with hair dressers, massage therapists, bar waits and strippers. Those professions tend to pay to work at an establishment.
There's also a certain vulnerability to the servers that a restaurant patron has, a waiter or waitress can do some horribly nasty things to your food without you knowing about it so tipping is prudent to avoid such pranks.
With a pizza boy, good service is by no means a certainty, he may not know the area well enough to find you and there's plenty of room for the service to improve or degrade, with these people, tips are prudent as it makes it more likely they will remember you and respond much more quickly.
To a certain extent, tipping is not a matter of being gracious and polite so it's not really a matter of etiquette. It's a facet of free enterprise and bartering with inherently selfish motivations as the improved service is likely to be at the expense of another patron.