How do I proceed when someone steals my mail?

I ordered some party supplies for my sons birthday. Somehow I entered the wrong mailing address; consequently, my neighbor received the package. The tracking number indicated that it had been delivered to their house on the 21st of October. When I confronted them to ask for the package they told me they didn't have it; however, his demeanor told me otherwise. I am confident they have it and are not giving it to me. It is not so much the money, being it was only $50, as it is they are KNOWINGLY stealing!!!! This is also a felony, but I'm not sure how to Proceed. I defiantly do not have a problem with taking this further; as I believe it is outrageous that they would steal. I even told them this was for my sons party on saturday, and they still will not give it back!