Great question. Authenticity is easy - I mostly deal with certified coins, which means they’ve all been authenticated and guaranteed by a 3rd party service which stands behind their mark (they will pay you for the coin if they make a mistake, which does happen).

For shipping, that’s just pretty standard across most industries - I have shipping insurance and if USPS fails, they’ll pay for it. But losses happen and it’s just a matter of business (thankfully they’re rare).