Thing is that signature required costs another $2+ dollars, and customers really balk at it, even the ones who place a big order. Also, they don't want to go down to the post office to collect it. and I don't blame them, since I've had to wait in line for 20 minutes to get a package myself. Some people have to go on their lunch or even leave work early to do that.
When the status shows delivered, I ask the customer to look around their house for the item or ask anyone they live with if they brought it in and forgot to say. Sometimes people with roommates will not get something and the roommate has it. Sometimes the carrier puts a package in a weird place. Worst is that the post office allows a carrier to leave a package with delivery confirmation in a "secure area," but this includes a mail room in an apartment building that only the tenants can get into. Thing is, some tenants will steal other tenants' mail if it is a package lying around. I have heard this often enough to believe it. People tell me this usually when they want the thing delivered to their work.
If the customer says no luck with checking into it at home, I ask them to talk to the carrier. Usually it turns out it was misdelivered. If the carrier says they left it or they can't get hold of the carrier, I call the delivering post office myself. Usually it turns up after that. But if it doesn't, I refund the person and tell them I cannot ship anything further to them because their address is not secure for mail delivery and that they should get a po box.