An easy way to think about this (for me at least) would be to think in terms of transferring liability to another party.
Who's responsible for the package/order when the package gets left with the neighbor? Would the liability/loss transfer to the customer if the package goes missing when the neighbor supposedly has/had it?
If you're a seller on Amazon/eBay and shipped the package yourself, the liability of the total value of the package is with the seller, unless you purchase insurance or signature tracking information.
For packages 13+ ounces (0.36+ kg), USPS automatically provides insurance up to $50 when sending packages Priority Mail (w/restrictions): http://pe.usps.gov/text/dmm300/609.htm