Wow, you know, part of me thinks that treatment like what happened to your mail deserves some kind of call to someone in authority at your post office. It's one thing if stuff gets lost for a long while or damaged--you can chalk that up to accident or negligence. But the post office is part of the government (Er, isn't it? It is where I live.)--unless people want to use a third-party courier for everything (which I doubt most people do), they don't have much choice but to use the post for all sorts of important, valuable, or private things, from personal letters to shopping to receiving/paying bills or taxes. Even if it doesn't always work well, it has to work honestly. We can't have thieves working in the post office.
Probably calling isn't going to do much on its own--it's only one call, and they probably wouldn't be able to track down who slit open your envelope, or prove it was done by a post employee on purpose. But it wouldn't hurt to have a (presumably less larcenous) authority figure at the post know that something happened. After enough complaints, there might be some cameras put in place or something. Unless there's some concern about retaliation or something.