You would need to check the regulations in your area. Sending some things through the mail could be illegal, if it could constitute either a health hazard or offensive material. Or if it could be considered threatening behaviour (such as sending funeral wreaths and dead flowers, which can be interpreted as a death threat). From the point of view of the sender, it feels like a fairly harmless joke. You KNOW you only intend it as a joke. But the recipient who gets it, who isn't "in" on the joke, could be wracking his/her brain trying to work out what it means, and we do tend to put the worst construction on things.
Just think about how we respond to a telephone ringing with nobody on the other end. The latest fad in telemarketing, is to have a computer "cold call" various phone numbers and the ones that get answered get noted and added to a database of "live" ones, meriting a human caller. We went through an experience of getting a computer cold caller on my mobile phone every hour or so, caller would hang up within the first ring or two. My phone would register "unidentified caller" and I began to wonder who was trying to get in touch with me. My mind was going crazy trying to work it out.
Another time, my phone actually displayed the calling number but I was getting the same regular calls. I was right next to my phone and was often surprised to find I had missed a call, it hadn't even rung! It seemed that as soon as the computer registered that the number was connected (there was no "call failure" automated message) that it hung up. I plugged the number into Google and it came up like a rash - someone trying to sell time share accommodation in Queensland. So I immediately began formal complaint proceedings. Meanwhile mother in law was getting the same calls and not only were they a nuisance to her, they were scaring her.
A few days later I managed to answer the phone - it did keep ringing, so I think it was my time to get a human caller. I scared the crud out of him by blaming HIM for all the cold calls, telling him I had already commenced proceedings for harassment and that if he personally was not responsible for the cold calls, then by continuing to work for a company that worked that way was putting him at risk for complicity in shonky and threatening business practices.
Sometimes we don't realise the reaction to the person on the receiving end, who has no idea what's really going on.
Mind you, if you are set up in a prank loop with a friend who knows it's you, that can be different. There can be a certain complicit acceptance of tis sort of thing, if it's going tit for tat.
And receiving manure in the mail is not necessarily a mean practical joke in all circumstances. One of my favourite authors form years ago was the Yorkshire vet, pen-named James Herriot. He described the jockeying for prestige in the practice especially with a young, attractive goat-lover. This girl would often send samples of goat droppings to the vet to ask it to be analysed for parasites. Of the three vets in the practice, it was generally the vet she admired most that she addressed the droppings to. So the vet who saw the jar of droppings (with the rest of the mail) at HIS place at the breakfast table, knew he had just received the highest accolade from his client!
Me - if I received a jar of manure and I could see a business card somewhere at the bottom, I'd probably just toss the whole thing anyway, card and all.
Or I'd empty it into the compost heap. I'm a farm girl, I'm used to manure. I could tell you some hair-raising stories about some fairly awful poop experiences... including some rat excretion stories that I can never dine out on, because people tend to leave the dinner table looking green!
(A popular science article I read once said you can dress a biologist up, but you can't take them out to dinner!)
Marg