Start with the reason this retail ecommerce software development company should exist, not a list of screens. Who will use this, how often, and what does the process look like without it? A vendor who knows what you are trying to achieve can propose a simpler way to reach it; a team that receives only the requirements as given can only price exactly what you asked for.
Describe the scope as user stories or scenarios: a walk through each important path. Just as important, write down what you are not building. A written out-of-scope list removes more friction during acceptance than the rest of the brief combined. Mark too which decisions are settled and which are still open — honest teams price those differently, and pretending everything is fixed only hurts you.
List the constraints. This means the platforms and services involved, the data you have and where it lives, regulatory obligations, traffic expectations, which devices matter and infrastructure that is already decided. Where a date is genuinely fixed, say why: an experienced team is usually able to rearrange the plan to meet it, but not if the date is a secret.
Say what completion means feature by feature. Clear acceptance criteria need not use special syntax: a plain-language note stating the expected behaviour is sufficient. This single habit shortens the sign-off process by a surprising margin and eliminates the usual argument at handover.
To close, ask for a specific format. Request a breakdown by feature or module, a written list outsourcing of software development assumptions, the risks the team sees and a range rather than a single figure. Take a broad range as information, not evasion: it normally identifies the part of the brief that needs work. Then clarify that area and ask again — the revised figure is the one worth planning around.