I paid for my mailbox and thus it should be up to me to decide who can put mail in it. But it is apparently against the law for UPS, DHL or FedEx to utilize personal mailboxes, and we are not just talking packages here. If they for example missed you at your house for a delivery that can't be released without a signature, they can't leave the notice in your box. Nor can they leave a copy of a shipping slip in there. That makes no sense to me.
It is my mailbox, on my property and I should decide who gets to put their grabby hands in there, not the government.