Some of my mail arrived already opened. What should I do?

It is not my bills or anything that comes in the small envelops. Some of my mail that comes in small boxes (samples and stuff) or large envelops have been showing up already opened. I don't know if it is my neighbors getting my mail by mistake, but they should not be opening it anyways. It might be the mail man, but I don't see why he would open it and then put it in our mail box. I do not know what to do. This started happening a week ago and just happened again today. I don't know if calling the post office will resolve anything if it is my neighbors doing it.