I do customization projects (the whole waterfall from requirements, specification and dev to docs, packaging, delivery and bugfixing) on a business application

I do customization projects (the whole waterfall from requirements, specification and dev to docs, packaging, delivery and bugfixing) on a business application (that is developed in a more agile team). The customer wants a specific custom feature and once they have it it should be done and over with the project. They want to know the cost and they want to know the delivery date (the actual person I am talking with is a sales rep who is talking to the real customer, so they usually want to know those things in reverse order, but that’s manageable). The well-known problem: The customer doesn’t know what they really want until they see a first iteration in practice. The solution: train the sales rep (by courses and experience) to join you in becoming an awesome author team for req and spec documents. Keep the scope small enough to be able to oversee all cross-effects of the new features. Keep the discussion alive with the customer to talk about these effects. Will this always work? No, I had absolutely insane projects out of everyone’s control. But it will work most of the time and if it did not the cake was just too big to swallow in one piece. Bottom line: If waterfall fits or fits not depends in my experience on the scale of the project.