E for estimable

How long will this story take to be implemented? The estimable principle leads to the most chaos misunderstandings when it comes to user story mapping. First of all, humans are terrible at estimating things. We usually estimate based on the best-case scenario, then add padding to handle the anomalies. Unfortunately, reality doesn't work like this. Unforeseeable things can and will happen, but that's fine. Estimates don't need to be accurate.

The shopping cart view should take less than half a sprint, depending on the team, while the whole checkout process might need to be split into multiple sprints. Always discuss estimates, with the development teams, even if you think you know them and their capabilities well enough.