The mailman can't leave the package on the doorstep the way she would in a rural area because the package may get stolen on a busy street. The mailman also can't leave the package at the front desk of the apartment building the way she might in an American city because the Dutch live in shared townhouses with no front desk to leave packages.
The package must be handed directly to someone, or it can't be delivered at all.
If a mailman comes to deliver my package when I am not home, she will ring my neighbor's doorbell next and give my package to my neighbor. Then she'll slip a note through the mailslot in my front door, telling me my package is next door. To get my package, I have to collect it from my neighbor's apartment.
This system works great, and it forces you get to know and trust your neighbors. Afterall, they're the ones in charge of all the stuff you order online.
In many little ways (like how they deliver packages), the Dutch create a sense of community with their neighbors which allows them to live and prosper so close together.
