Maybe it would be enough to merely ask the customer if the flowers had already been paid for – sounding like a gently reminder just in case, and also sounding like you would be totally willing to take their word on it if they say yes. Asking for a receipt implies that you would NOT take their word and need proof, or else you would make them pay even if it might possibly mean paying for it a second time.
BTW – a few very rich people probably really do own the world.