We don't believe in fixed-bid projects. Especially for startups, they make no sense.
We understand that your requirements may change from day-to-day. It's a waste of your time and our time to argue over whether something is "in scope". You need developers who can prototype quickly and who can adapt to rapid change. Our whole approach is designed to facilitate this.
You buy blocks of time from us then instruct us week-to-week how they should be spent. For example, you might buy 20 hours and ask us to build an initial prototype. Then buy 100 hours to get a minimum viable product to launch. If half way through those 100 hours the requirements change: no problem.
We use GitHub as our code host, JIRA for bug tracking, Trello for tracking progress on stories and Openshift for rapid deployment of prototypes and development branches at every stage.
If you're interested in finding out more, contact at firstname.lastname@example.org