permission to open other peoples mail

Factual Questions
1

I´ve seen that you can get a green label to put on your box that you´re going to send by mail. This label says that the custom people are allowed to open this package and examine the content.

I hear that you MUST put this label on things going to the USA, but what about other countries? Is it the same everywhere?
And do they actually check every package that comes from another country?

2

They spot check. I believe any mail traveling from one country to another can be checked by cusoms. I know that you have to fill out a green customs declaration form on any packages going from the U.S. to other counties.