The short answer
A proposal usually moves through fit, diligence, defined scope, contract and delivery. Payment may depend on acceptance rather than the initial expression of interest.
Distinguish the milestones
Label indicative estimates, binding prices, sample acceptance and final acceptance separately. Ask which event starts the payment clock.
Control what changes
If new exclusions or preparation work emerge, confirm the revised scope and price in writing. Do not treat an early number as unconditional cash.
What to record before deciding
Keep the written scope, official provider source, unresolved questions and the person responsible for each check. Treat a marketing claim as a claim until it is reflected in the agreement. A directory entry does not approve the transaction or establish your rights to share the records.