Open with the business problem, not your preferred technology. Who will use it day to day, how often, and what happens today? A vendor who knows what you are trying to achieve will suggest a simpler way to reach it; one who only sees a list of screens will price your assumptions along with the work.

Set out the scope as concrete flows: a walk through each important path. Equally important, list what is out of scope. An explicit list of exclusions prevents more argument during acceptance than almost anything else in the document. Indicate as well which parts are firm and which are still under discussion — honest teams price those differently, and pretending everything is fixed only hurts you.

List the constraints. These include the platforms and custom software development services involved, the data you already hold and its condition, hire ecommerce developers regulatory obligations, user volumes, which devices matter and stacks you cannot change. If there is a hard date, explain what drives it: an experienced team is usually able to rearrange the plan to protect it, provided they hear about it early.

Define what done means for the important items. Acceptance criteria do not need formal language: a short paragraph stating what must be true when the feature works will do. This one section compresses the review at the end considerably and closes off the most common source of disputes.

To close, ask for a specific format. Ask for a breakdown by feature or module, a written list of assumptions, whatever the team considers risky and an optimistic and a pessimistic figure. Read a wide range as a signal about the brief: it tells you the part of the brief that needs work. Then tighten that section and request a revised number — the second estimate will be the one worth planning around.