You can do fixed delivery, variable features or fixed features, variable delivery. It doesn't matter which, the point is just to acknowledge that you can't fix

You can do fixed delivery, variable features or fixed features, variable delivery. It doesn't matter which, the point is just to acknowledge that you can't fix both so you better understand which is more important to you. That said, it's perfectly reasonable for customers to expect to have some promise about what they're paying for. That's where you trot out the idea of a minimum viable product: Make some tough decisions about what features are actually required, and then agree to a timeline that makes it a virtual certainty that you'll be able to deliver all of the required ones. When you hit that mark ahead of schedule and start burning through the list of nice-to-haves they'll be elated.