You must be at a PO box site, USPS drops things off at residential addresses the same as UPS and FedEx.
We had a lot of issues with USPS when they first started working with them - things not getting to us on time, mostly. They have really improved things since then, and a friend of mine who works for USPS has mentioned that they have made some pretty massive changes specifically because of Amazon's business. They even began Sunday delivery services specifically because of Amazon, when before they started working with them they were talking about cutting Saturdays.