An estimate that arrives instantly should be treated as a bad sign. Any serious team responds with clarifying questions before any number: about integrations. A supplier that quotes without asking anything is probably working from a template, and that guess will be corrected later — on your budget.
Look out for a mismatch between the team in the pitch and the hire react native developers actually assigned. Ask for named engineers in the statement of work, with a provision that requires notice before anyone is swapped. A team that will only describe a pool of resources and never names people is reserving the right to assign anyone it likes.
Require the source repository from the start. A provider that hands over a build only at the end of each phase expects you to take delivery on faith. Visible commits tell you who is really on the project far better than a slide deck. This extends to the CI pipeline: if it does not exist, assurances about quality are just talk.
Ambiguous phrasing around IP is not an oversight. The contract must state plainly that the code, designs and documentation transfer to the client upon settlement of the relevant invoice. Check also the governing law and software development company in united states the payment schedule: a large upfront payment with no milestone tied to it takes away the only leverage you have.
Finally, pay attention to the working rhythm. Establish what overlap there will be each day, who is expected to answer questions and on what response times. Four hours of overlap is usually enough; no overlap stretches a five-minute question into a lost day. Sloppy written English in the proposal does not improve later.