First of all, tampering with mail is a felony offense.
The next time you get a package with the corners torn open (but not containing meds), I wouldn't accept it, but ask to speak with the local postmaster or manager of the post office about it. In other words raise holy heck about having your mail tampered with. If they don't do anything, I would file a complaint with the postal inspectors office. It might also be helpful to see if anyone in your town has had similar things happen to their packages or packages that don't arrive. It could be that someone is stealing mail or looking for something in the mail to steal.
Here is a link to a search page to find your closest postal inspectors office:
http://www.usps.com/ncsc/locators/find-is.html
Now, about your original question, I really don't know, but I suspect that they would be good if you had a specific plan, such as rent one in a different town. I'm sure you have to provide ID and whatnot to get one, so it would still have your name on the packages.