There are a bunch of issues to consider:
1. If the tracking shows delivered but the buyer states they did not receive the package, I do everything I can to help them find it. I let them know to call their local post office (I provide them the phone number, which I can get by googling "Post Office" and their zip code) and tell them to ask to speak to the carrier who scanned the package as "delivered". I give them a description of the color and dimensions of the package and the type of packaging so they can describe it to the carrier. If they call within a day or two, carriers often remember specific packages and can sometimes figure out if they've mis-delivered them.
Keep in mind, the post office DOES occasionally screw up - a buyer is not necessarily running a scam if the post office says "delivered" and the customer says it's not. Also, some postal carriers scan all their packages delivered at the beginning of their shift - so at 8 a.m. the buyer may freak that the package shows "delivered" when it won't actually get to him until 4 p.m. And then sometimes, the carrier runs out of time on his route and doesn't deliver all his packages that are already scanned until the next day.
2. If the package is "lost in space" (no recent scans to tell you where it is), you can get the current tracking at USPS.com and ASK FOR EMAIL UPDATES (there's a box to click). You can ask for updates just to you, or to you and your customer. I don't know if I've just been lucky, but EVERY time I've had a package that the post office just vanishes in a puff of smoke and is stalled on tracking somewhere for days and days and have used this service, the package miraculously re-enters the flow of packages within a few days. Like I said, could be luck, or maybe they have to explain "never arrived" packages in some kind of internal auditing that this track-trace function generates.
3. If the package never does scan delivered, you will be refunding under both eBay and PayPal rules. This does not mean you should insure every package you send - for most sellers, that's a spectacular waste of money. Insure what you cannot afford to refund - for some folks, this "pain point" is $50, for some $1000. Bottom line, if you put a dollar or two in an "insurance jar" on your desk, you're going to find at the end of the year that you have a large surplus of cash even after a refund or three.