This is more of a code submission more than a support question.
For everyone else reading this:
This is not official code. I'm posting this to submit it to @Barry but it is still his call to integrate it into the plugin as an official release.
I've modified the Marketpress plugin for Affiliates to enable paying the affiliates a fixed amount for each Marketpress sale by adding a checkbox below the Affiliate Settings in Marketpress payment to indicate that the amount is a fixed amount not percentage.
I'm attaching my changed affiliate/affiliateincludes/addons/marketpress.php file and diff of the changes.