When I mailed my used filter cartridge to a member on here, I didn't tell the USPS what was in it. I stuck it in one of those white-locktop ziploc bags and wrapped it further in a bit of bubble wrap in case the bag broke and it leaked. Then I taped up the bubble wrap and stuck it in a box. So if it had leaked, no one would've known. It arrived just fine.
I know the USPS asks you if there's anything liquid, flammable, etc. Just say no.
1 yoyo loach, 2 polkadot loaches, 6 cherry barbs, 2 turquoise rainbows, 1 chinese algae eater, 2 harlequin rasboras; java fern, java moss, hornwort, Asian ambulia