Thereof, is it against the law to put mail in someone's mailbox?
It is a crime to put anything in anyone's mailbox, with or without consent. Mailboxes are considered federal property and tampering with them in any way is a federal crime. It is also illegal to place items in your mailbox with the intent of someone other than the mail carrier to retrieve it.
Likewise, can I put a note in my neighbor's mailbox? It's not quite common knowledge, however, that it's illegal to place a letter in a mailbox without proper postage—and yep, that goes for dropping off a party invitation over at the neighbor's house. Even more surprising: The postal service is actually trying to enforce the rule.
In this manner, how do I send mail to a PO box?
Print the USPS Form 1093 application and complete by hand. Take the form to a Post Office that is convenient for you. Once USPS verifies your address and receives your payment, they'll provide you with your new PO Box address and begin your service.
Can you have a PO box as your billing address?
So, yes, it's possible to use a PO Box for the bank, but you will also have to give them an address AND their systems will have to support a separate mailing address from the physical address.