Each week in the roadmap has its own milestones, so it's clear what should have been delivered before the next week is invoiced. Problems surface early because updates are daily or near-daily in a shared Slack or Teams channel, and everyone involved on your side can see them, so issues come up within days. Weeks are paid upfront, so a pause takes effect at the end of the week already under way. If the project later needs more time, the contract can be extended by adding weeks.