Sending toys by media mail is considered mail fraud, and it's a federal offense. I doubt if they caught you they'd prosecute you, but it's not worth the fine and the possibility of having it confiscated.
Now, a couple of minifigs should absolutely not cost $40 to send. You're doing something wrong here.
Typicaly when mailing something small, a bubble mailer will suffice, and you need a customs form. It's a little green sticker that gets put on the package where you have to say whats in it and what the value is. See here
- but don't try and print this and use it. They're free at the post office anyways. "LEGO Figures" is sufficient description, and $5 is reasonable to put even if they're worth more. If it's lost you wont be reimbursed so you don't need to worry about being exact, and the lower you put the lower customs fee they'll have to pay on their end. And I always mark mine as "gift" because that's the closest category offered.
Most important thing about sending it is to make sure you have the address written correctly. Print it and put tape over it so it can not smudge if you can. Then just make sure the envelope is fully closed and you should have no trouble.