@celtish I run a service business and charge hours used. My proposals also have a range, and I come within that range. I dont get the issue you do, but if I did, then my response would be politely "My invoice is well within the estimate range in the proposal, so I will need to charge for the hours used" and leave at that.
It sounds like these customers are just trying to push any button they can to see what the limits are. I would politely not entertain them. That is, dont entertain them in the game they want play. If you dont respond, they might just stop.
Also, I am always advising the customer in ways they can save money, so if they then pushed back more, I would want to remind them that, compared to my competitors, I have made several suggestions in other parts of the contract where they can save money, and in fact I am on their side (to make them feel a touch guilty).