https://www.railinc.com/rportal/documents/18/260641/Guidefor...
The software for it has to work. There will be months of work implementing the rules engine before you have anything to show the customer besides a mock-up..
You won’t really need two week sprints to get feedback from customers. It either implements all of the rules or it doesn’t. I can imagine the same happening for tax software.
If a customer isn't committed to the project, then I'd worry about committing to it as well. Even if the contract guaranteed I'll get paid, there are few things as demoralising as a project falling over because, once push came to shove, the customer didn't even have the manpower to deploy your product. Those are usually the same people who would do the evaluation, so with my experience, as an employee, if they're not there, I'm going to look for a project where they are — regardless of how confident sales might be.
"This rough implementation is how I (we) envision this feature working. What do you think?"
"We've got the feature working well but it requires non-trivial back end changes. Would you be willing to wait or will you be satisfied if it works but only in a very manual way until we get the back end changes in place?"
Week 2 after release: "The feature is live... What do you think? How is it working?"
These are conversations the developers should be having with the end users. Regularly! It's when developers only ever talk to their boss and/or the project manager that pushes development into the anti-user zone.