Yes, it most certainly is, going through some one's mail is illegal in any civilized country.
Yes Yes
It is a crime to send any illegal substance through the mail.
Yes, it is illegal to send a glitter bomb through the mail as it can be considered mail fraud or a form of harassment.
It is not illegal to send money through the mail internationally to your friends and family. You should never send cash through the mail.
No. Your parents and guardians can exercise wide latitude over you.
Yes, as long as it isn't stolen mail. No, it is illegal, even to place a flyer on the flag. The mailbox is US Government property.
No, it is illegal to mail controlled substances.
No.
If they don't have your permission, it is entirely illegal.
Yes, it is illegal to go through someone's mail without their permission. This is considered a violation of federal law and can result in criminal charges.
Yes, from the time the mail is sent until it is received by the person it is sent to the mail is the property of the post office and therefore government property. Preventing a person from receiving their mail is theft of government property; for which you can go to prison.
Yes. That's a biohazard.