Perhaps Tupperware isn’t allowed, but zip-lock bags are apparently OK packaging for manure sent through the mail.
A simple bit of googling shows plenty of companies offering to sell various kinds of manure with shipping via U. S. Mail. It’s most commonly offered by businesses catering to mushroom growers who only need small quantities for use as fertilizer. BTW, horse manure seems to be preferred for mushrooms.
UPDATE—OK, let’s assume that the Cabin Boy™ files a complaint with the Postal Inspection Service. So what? The only result will be that there’s one more government office where eyes roll when they hear the name Schmalfeldt.