Interestingly, Amazon is changing the terms of their Associates program to (among other things) specifically prohibit browser plugins from being eligible for sales commissions:

  7.	Except as agreed between you and us in a separate written agreement 
  referencing this Participation Requirement, you will not use any Content or 
  Special Link, or otherwise link to the Amazon Site, on or in connection with:

  a.	any client-side software application (e.g., a browser plug-in, 
  helper object, toolbar, extension, or component or any other application 
  executable or installable by an end user) on any device, including 
  computers, mobile phones, tablets, or other handheld devices;

- https://affiliate-program.amazon.com/gp/associates/help/oper...