Minimum wage for tipped restaurant workers and fast food workers is different. Tipped restaurant workers make around $3.25/hour and make the money off the tips. Fast food workers make the state/federal minimum wage of $8-15. Tipped restaurant workers typically do not see a check, it is $0.00 after taxes so they survive on your tips. Very, very different. Literally making this profile after being on this forum for months to try to inform you of this because it's important for you to know. Whether you believe it should be this way or not is another discussion but it's important to know how it actually works.
My sister spent 6-7 years as a server in a restaurant, so I am familiar with this. Like you said, whether it "should" be like that is another discussion. I personally like places like Japan, which doesnt really have tipping at all (except for places that deal exclusively with tourists, and only then to not confuse / offend them).
If you ask a native Japanese person about tips, they are confused, because "the price of the product includes good service, so what are the tips for?" People performing those jobs make a better wage there than similar places here in the US, I was told. So refreshing!
Anyway, I have never understood why, for wait staff, the burden of paying the persons salary is on the customer (tips). As for "tipping my mail carrier", I order a lot of things online (and did pre covid, as well). Many of my packages are delivered by my regular mail carrier. I know this, because I have security cameras for my porch and backyard.
Anyway, my mail carrier always goes out of the way to tuck my packages behind a pillar, or just inside my back gate, etc out of view. I live on a street that happens to be one of the main shortcuts through my city, so I get a fair amount of traffic on my street. Anyway, I was greatful that my carrier took that action to "hide" my packages, without any input from me, all year. I thought a thank you card, $50 and a note inside thanking them for doing so might go a long way in letting them know that "someone noticed, and cared" that they were doing something a bit extra.