Which Postmaster? Santa Monica or yours? I would complain to the Santa Monica distribution center, since that's where it passed through. May not get anywhere,but all letters are photographed, so they may be able to tell you whether or not it arrived and/or left the DC intact.

It's not an extremely common thing, but it happens sometimes. Everybody will tell you that they're not responsible and the mail goes through so many hands that you can never know if you're talking to the person who took it or not. You could file a complaint and if many people complain then someone might look into it.

Something I saw advised on a forum once was to make the envelope as boring as possible. So no "Photos-Do not bend" or anything like that. It was a postcard forum and they're thicker than photos, so you'd have to consider which risk is bigger.

Postal workers DO steal items (don't send w/o tracking to France or Italy , learned the hard way)

Today I received what I assume would have been a success. My SASE came back, but somewhere along the way it had been torn open and the contents removed. Despite the Santa Monica post mark, there was no return address, so I'm not sure who this would have come from. According to the post master, it isn't something that happened in their care and they hold no responsibility.