In the end of December, I recommended Amazon FBA Calculator as a great extension to more quickly figure out fees for particular listings. The actual tool is great.
A few people have brought up the same problem with the tool. The extension hijacks your links and adds in a referral link so that they get paid a referral fee whenever you buy something on Amazon. Not just when you use their tool.
When you go to Amazon.com, you will see that the link changes to http://www.amazon.com/?tag=dailydeal0c6-20
That add on to the end of the link is their referral link.
I don't have a problem with them making money. They are providing a service. However, I would have liked to have known that. I don't know what happens when you click on someone else's referral link since both referral IDs seem to be on the link. Who gets the commission?
Either way, now you know and you can make a decision on what you want to do.
I haven't currently tried out any other similar tools. If someone knows of a plugin that doesn't have this problem, please leave a comment.