This may be a mean thing to say - but do your neighbors rifle through your mail? We have a box on the sidewalk, our unscrupulous neighbors (mostly their kids) used to take my things all the time. Proof? My mom would have to go to their house to get back report cards, checks, comic books, etc. Usually opened/written on/damanged
One thing that may help is adding the extra four digits at the end of your zip code to your packages if you don't already. Each home has one, not sure about apts, etc. It's specifically is for your house, not just your neighborhood and cuts down on mailperson's error.
Things "lost" in the mail: both presidential absentee ballots, star wars x-wing 1/2 from Wizard comics, various cereal sample boxes, 4 plant shipments, 2 amazon.com packages (luckily amazon will resend these), 4 report cards, 10+ checks, billions of bills, 2 college rejection letters, 1 package of sea monkeys, various magazines (except for National Geographic, I guess only I like that), lots of credit card offers, 15+ personal letters (thank god for email), and... much more!