You need a mailbox by federal law. $2000 penalty for not having one and no one but the us postal service can touch your mailbox. So no dice there.
180's are simple to figure out in terms of what they are. They are much much harder to execute.
If you are the one who starts every conversation...stop. If you are the one who calls....stop. If you are the one who does things for him, like gets the card and gift for his parents and family for special events, stop.
But if you are the one who pays the bills, you have to keep doing that. If you are the one who figures out the logistics for your son, you have to keep doing that for your son.
But anything that benefits him, and only him you can stop doing. He's saying he wants out and wants to live his own life, let him.