[we treat customers with gold]
WHERE: At the car wash used by one of the authors of I Hate Duane Reade: Service from Hell (and nice non-capitalization of your preposition, by the way).
CORRECTION: You are
GRAMMAR: This one took a little creative correction because the meaning of the original is somewhat vague. After all, the first sentence is not grammatically incorrect, in and of itself. If the author wishes to indicate that the addressee, singular or plural, is both the one and only prince and a group of princesses, then the sentence is in fact perfect. (A more realistic example: The priest and congregants entered the church.)
However, I think it's safe to say that the car wash management does not really believe that each individual who reads that sign is both the one and only prince and a bevy of princesses. It's a metaphor, obviously, and meant to indicate to all customers that they will be given the royal treatment. As such, it's necessary to create an agreement in number, either plural or singular, and I opted for the plural. Once you've got that, there's no longer a need for the definite article, so we can toss the like so much Rainex down the drain. I can see how these mistakes got made — the is a perennial doozy for non-native speakers, and both prince and princess, with their S-sound endings, raise confusing questions about whether they're singular or plural.
The second sentence is easier to deal with. Treating customers with gold means using gold to do something to the customers, and it has an unfortunate medical connotation that I would not welcome even at a car wash that considers me the heir to a throne. (We treat customers' cars with wax would be more appropriate.) Instead, I assume they mean that they treat their customers as one would treat gold: with respect for their great value. It is a fine sentiment once rendered correctly. And I threw in our before customers because it improves the flow, and anyway I doubt the car wash guys treat, say, all Duane Reade customers (to choose an establishment at random) like gold. Clearly this is about their customers, who are special — little baby kings and queens, in fact.