there are a few things they look for when plants are shipped from one country to another. 1. is it contraband.. (IE is it illegal) It might be possible to ship say a Lophophora form one country to another legally. But say in America it's illegal. If you ship one and it's examined it will be seized. 2. If it is a noxious weed. Shipping Opuntia to Australia as an example WOULD be a bad idea. 3. is it in soil? Soil sent from one country to another for some reason (mostly pests/diseases) is frowned upon. 4. Lastly. Does the state in the country (IE the US) allow that plant to be imported. In some places they protect plants either native ones or agricultural ones from disease, pests or competition... As to what would happen if person A. sent person B. plant C, who knows. Might not be a problem, or it might get seized. I have no idea what would happen between the countries. (oh and sometimes they require certificates.. I am not an expert on this, but I seem to remember some issues in the past...)
My advice is to declare what you are sending and basically talk to the post office. Here in AZ they DO not care that I ship, nor receive cactus at all. I even told the guy working there that it had cactus inside and he said it was fine.
good luck if you trade. To me it's sad when cacti gets thrown away so..
Here's to you, all you insidious creatures of green..er I mean cacti.