I've worked with clients in a similar way, but without the commitment portion of scrum. We'd discuss priority order, scope, estimates and then bill hourly. Worked very well.

IMO fixed price features aren't good for either party. Your customer doesn't/shouldn't want you to cut corners or get sloppy if things are more complex than expected. If the customer doesn't like the incremental work delivered, they should end the engagement. To set expectations better, you should have an initial scoping to understand what they are looking for and ideally give you some time with three codebase to understand what you are getting into