522.3, Packaging and Markings: "A mailpiece marked “If Undeliverable, Abandon” may not be accepted for mailing." In other terms, you cannot put a dead animal in a box, then give it to the UPS for them to dispose of. I wonder how many people abused this particular "loophole" before they closed it.
Surely it's cheaper to dispose of it with your other garbage, or take it to a local dump, than paying postage to "mail" it?
But yeah, when they introduced that fee, my wife's family just started burying newborn lambs that didn't survive, rather than, y'know, fedexing them to people.