If your customer says something you disagree with, and you have valid arguments, then challenge them.
Not their authority, not their past decisions, not their role in the decision, no. But their preconceptions, their misunderstandings, and their prejudices are fair game.
Give them a mental challenge. Spar with them.
You’re an expert in your domain, and they’re an expert in theirs. You’re at par. But you probably know more about their domain than they do about yours. Advantage you.
You’re asking them to make a huge investment and business decision. Let them know what you’re made of. Take a punch and punch back. They’ll respect you for it.