Good afternoon and welcome to JustAnswer,
Sorry to hear that your package was not delivered to your apartment. Our site is not connected in anyway with the USPS but what we can do is offer information on how to contact their office to get your package sent to you.
My first suggestion is to contact your local post office branch as I would believe that your package is sitting in their delivery room. Sometimes the package would be sent back to the seller but since you contacted them already, your package I would bet is with your local office.
The way in which to locate your package can be found in this link I have provided as I believe the information listed can be of great service to locate the missing package:
I hope that the information provided was found helpful as please let me know of any additional questions/concerns you might have. Also if you could take a second to rate my assistance so that I know my help was found useful today.