Let me answer that for you.

Gold customers will never come again until very late in the game.

They appear only when the non-gold non-closer customers left are less than the amoount required for the day. When that happens, a random Gold Customer will pop up (and surprise the toppings out of me, in my case) and take his/hers usual order.

When they would normally progress in rank, you instead get extra points and tips, and the remain gold customers.

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