Also please consider competition for cargo space. I've "next day air" shipped things to avoid checking bags because, hey, why bother with the hassle? And for non-international stuff I don't need soon (dirty socks?) I've ground shipped at the very cheapest rates. I've walked on to planes and trains with little more than a tiny bag for camera and tickets and phone and such a couple times. Its a nice way to travel. Not as nice as paying for 1st class, but not as expensive, shipping is well worth the money.

The problem is that cheaper air shipping means it may very well be in the same plane I'm flying on, insane as that sounds. Then multiple middlemen all trying to get their cut, etc.

Shipping is infinitely more reliable than airline baggage, which is weird to me. The difference in loss rates is like 100 to 1. Everyone knows someone who ended up with missing baggage if not themselves, but its incredibly rare to lose a tracked postal package. I really don't understand this beyond maybe baggage handlers are more likely to be thieves than USPS mailmen.