I feel that it's almost always necessary to have more than one delivery at different stages otherwise some important feature or bug fix doesn't make it in.

You could say each increment is a fixed partial fee or milestone but that doesn't work unless there is adequate requirements analysis at the start, and they are small increments that can be determined dynamically and/or in a highly prioritized way.

And also each increment has to be the equivalent of billing for that block of time anyway.