Didn't see him mention this anywhere in the documentation, so thought it was worth telling people about.
On line 311 (at least on version 1.3 of this plugin), the author includes code that will randomly change the AmazonID in the URLs generated from yours to his, thereby skimming 10% of your potential sales.
Normally, the URLs include whatever AmazonID you supply in the setup, but if a randomly generated number between 1 and 10 turns out to be a 6, his code replaces your value with his own.
Now, I apologize if that's stated somewhere and I'm just not seeing it, but to take 10% of your users' Amazon traffic without telling them just ain't right.
It looks like you can safely comment or delete that line if you prefer not to tithe to the author. Or, if you do feel like you owe him a little somethin' somethin' for the work he put in but don't think 10% is right, modify that line to a different value.
The line looks like this:
if (rand(1, 10) == 6) $associateId = 'subversionfor-20';
And it's right under this comment:
// 10% lovin'