Not really. You can bid a project-priced project with T&M overages, which is what every project at Matasano did; Matasano was founded by @stake people, which is where we got our project structure from; @stake was Cambridge Technology Partners people, which is presumably where they got it from; and so on down the line all the way back to, I assume, Samuel Adams or Patrick Henry or some other famous Bostonian who came up with the concept.

Just break the project out by week (or by day if it's short) and provide a "total project price" line, and then a clause saying you'll charge overages at a pro-rata day rate. Of course, you can decide whether or not to actually charge OT; if you're OT because of something you did wrong, you're best off not charging, while if the client did something flagrantly wrong, like not kick off the project until 2/3rds of the ostensibly billable time was over, you're well within your rights to do so.

This stuff isn't hard, and I've been a little confused for, like, about 17 years as to how message boards manage to make it seem complicated.

If this is news to you, and you start doing it yourself (you should; it's easy, aligns incentives, and will probably make you more money) you have to credit me, but also cite the whole Apostolic succession (Matasano, @stake, CATP, ..., Paul Revere) I just did.