IMO the only mistake was sending the bass detuned, AFAIK there is no reason to do that, and no manufacturer I know of does so. But nor do I believe that would have caused the bass to go from low sizzle to high action.
You are not responsible for providing a setup, or second setup, to the customer.
My only 2 cents is, if you keep selling instruments, don't ship them set up that way (which I happen to like aslo), as there is no clear definition for this purpose between "sizzle" and "buzz". I would return it to factory spec action, or just raise it a touch and put the action measurement in the listing, not words like "low" or "medium".
Best of luck. If I were you, I would not respond to any more messages from the buyer.