An estimate that arrives instantly should be treated as a bad sign. A competent team returns questions first: about integrations. A supplier that commits to a figure before understanding the scope is simply guessing, and that guess becomes a change request later — on your budget.
Be wary of any distance between the people you meet and those who eventually appear in the repository. Ask for flutter development services specific people rather than roles in the statement of work, with a clause that requires notice before anyone is swapped. A vendor that only offers a pool of resources and never names individuals is reserving the right to assign anyone it likes.
Ask for the source repository from the first week. A provider that shows nothing between demos expects you to trust a black box. Regular commits and pull requests show you how many people are really working far better than a slide deck. The same applies to the build and deployment setup: if there is no pipeline, promises about quality are nothing more than words.
Vague wording in the contract around code ownership is not a formality. The contract should state explicitly that all deliverables transfer to the client on payment. Check also which is better monolith or microservices country's law applies and the payment schedule: a request for most of the money up front with no milestone tied to it eliminates any leverage you would otherwise keep.
Lastly, look at communication. Establish what overlap there will be with your working day, which person handles your questions and how quickly. A few hours of overlap generally works; no overlap converts a five-minute question into a twenty-four hour round trip. Careless writing in the proposal will not improve once the work starts.