explanation of what this means - don't tell your customer he is king then treat him like a peasant?

Answer 1

treat the customer good, in fact "like a king"

especially after promising the customer

such good treatment and excellent service

IN other words,

do as you say and keep your promises

Answer 2

It basically means don't tell the customer they are important and valued while your actions and attitude indicate they are worthless - make your actions match your words.