I only use USPS for international orders and tell the buyer that the package is out of my control. I will guarantee nada.
For US customers, I force the to use UPS. The few extra dollars added to the tab give the person a real tracking number and insurance is free for shipments under a $100.
As a general rule, I have just accepted that I will have a smalll percentage of crazy people. I have one that paid for an item five days late and then filed a claim on day seven. She thought I was supposed to ship as soon as she bought...not when she paid.