I wrote a plugin (“AmazoLinkenator”) that automatically adds your Amazon afilliate link to all Amazon links – overwriting any existing Amazon affiliate link that others might place in a comment. The techniques in there could be added to to do other affiliate places.
Found here: https://wordpress.org/plugins/amazolinkenator/ It has options to shorten via Bit.ly your links, so you can obfusticate your Amazon affiliate tag. (You’ll need a Bit.ly key; instructions are on the plugin settings page.)
It works on all content when saved: pages, posts, comments. Since it is open-source, you could adjust the code for your needs (making a copy of the plugin and renaming it, so that any changes you make aren’t overwritten by a plugin update).