I recently bought a new car, and I left my mailbag, a blanket, a backpack and some clothes in my ex's truck so I could trade in mine.  This was a Friday.  I forgot to get them back, but was supposed to get them 3 days later when I would see him again. He was visiting his mother.  (We live in seperate states.)  Something went down at his mother's house and she kicked him out so he left to goback to his state. 

Well, he mailed me the mail back, 2 pairs of underwear and my medicine.

The mail however, was all opened.  He had even opened something that was marked return to sender - something from the Disney Movie Club that I hadn't ordered.  So now I have to pay $30 for it.  What is my recourse?  Is it illegal for him to do that?  I definitely didn't give him permission to open my mail, and now I have to pay $30 for a movie I didn't want.  Also, I bet he's probably not going to let me have the blanket, backpack or the rest of my clothes either.  I guess that's probably moot, but it pisses me off.

What can i do, if anything, about the mail?

I believe it is - but I don't know who and how to report it or how you can prove he opened it!  How long before you rec'd it in the mail?   Did he send it back or did someone else.  Maybe it is a new girlfriend, family who actually opened it and he didn't give it a second thought?

Tampering with mail is not only illegal, it's a federal offense.  Since it wasn't removed from your mailbox tho, not sure of your options.  Check with your local police dept. if it really bothers you, but since you left them in his vehicle...dunno, may not be considered 'mail' until you deliver them to USPS.

Because I have so much time on my hands and I am trying to keep my busy and off some stuff - I called the postal service ;)

They said:  Once it leaves the postal stream...it has been delivered then it is out of the postal service hands to the person who it was to be delivered, it becomes a local issue -police department or DAs office and they have no idea of the laws.  It is illiegal (Federal Offense) if it is still in the postal stream. I think if he took the item you planned on returning or $$ out of the envelopes than it is stealing.    So it looks like as soon as the PO hands off the mail to you it is governed by different laws.  Hope this helps. 

You might still be able to return the movie you'd just have to pay shipping.  I've done it before when I've opened them and then realized it was someting I didn't order.  It might be worth the $3 in shipping to get the $30 refund.

According to "Free Legal Questions ...Ask a Lawyer a Free Legal Question" 's Brent Rose: It violates federal law to open another person's mail.  (Of the The Orsini & Rose Law Firm in case anyone wants credituals on the guy).

At one time, I was informed that if it was mailed to you without your request, you are under no legal obligation to pay for it.  I see no reason why you can't still return the item, opened or not.

Oh, you can still return it, you'll just have to pay for the shipping.

It can only be marked 'return to sender' and sent back at no cost to you, if it's not opened.

They said:  Once it leaves the postal stream...it has been delivered then it is out of the postal service hands to the person who it was to be delivered, it becomes a local issue -police department or DAs office and they have no idea of the laws

This is what I was thinking. Since you left the mail in his custody I doubt you can do much about it.