Hmm that is very odd. I know that when I post to Canada from Germany I have to have a return address on the outside plus a CN22 label saying what is inside. I usually write gloves or socks or something like that. All my packages have got to their Canadian destination but I have to admit it does take longer than anywhere else I post to in the world for some reason.
However, I would never post anything that has scat or bottles of pee etc., or the like outside Europe, its almost a definate chance it will be stopped and checked.
Are your packages being seized at the “country of destination” border or the US border, just out of curiousity?