I don’t know what I can tell you that isn’t already in what I’ve linked. Yes, it’s not fair but you offer multiple revisions and whether it’s a change or a new order is subjective. You need to offer 2 or 3 revisions.
If this was me for this order, I’d tell the buyer very politely that he has one revision left, or he’s just used his last revision and if he asks for another, send a custom extra in the order for the revision and redeliver the last file. I didn’t check but if you say anywhere that you offer unlimited revisions then you can’t do this and you may have to cancel.