Take a picture(s) of the item being sent. Make out two address labels, one with a description that goes inside the box.
Put your name, address, phone no. and email address INSIDE THE BOX, along with the receivers name, address, phone number, and include a description of the contents. Why, because there are a number of reasons a package will get pulled and sent to the Dead Mail Drop, where it will stay until it is eventually opened.
If the shipping label gets damaged and they can't make out the sender or receipient, or if the item falls out of the box, or the box is damaged it might end up in the Dead Mail Drop.
The USPS tracking system gives them more info than what we see online.
Call or go into the USPS office you shipped out of and talk with the manager, get him to file a report and give him a full description, and picture(s) of the contents.