Sorry this is happening to you, but..
Why wasn't the first thing you did call the police?
As you said, messing with the post is, I read once, a federal crime. And if not, it's still vandalism on a large scale.
A question: you are talking about those mailboxes that are placed near the curb like I see in movies all the time, right?
I never got that. Here in the Netherlands, and in Germany e.g., the mail is deliverd to mail-slots INSIDE the frontdoor or the building itself. No breaking them open without committing braeking & entering at the same time.

But, as the post had been dilivered into the mailbox, I don't think the postmaster has anything he could or would do. It's not his job, his buck stops when the post is delivered.

So...go to the police..