Alarming number of customers claiming “Tracking shows delivered, but I didn’t get my package”

I ship most FBM orders via USPS First Class. Lately, I have dealt with a concerning amount of orders where USPS tracking marks the package as delivered, but the customer claims they did not receive anything and demands a refund or replacement. How often should I expect the Post Office to make an error and fail to deliver the package? I would estimate that I deal with this issue for 4-5% of my Amazon orders. But it almost never seems to happens with orders directly from my website. Is this just a case of dishonest Amazon customers, or that truly how bad USPS is? I’m working on moving toward FBA, but in the meantime this is a rather costly issue.