How much are you really worried by this? Amazon know my purchasing history for over 5 years and the best they seem to do is show me offers for things similar to what I've already bought which I'm not going to buy again for a long time.
I admit seeing brands and advertising will introduce a bias but I can't see how it would influence me much when for important/expensive purchases I usually do a lot of research first.
I don't love advertising but if it's only unconsciously influencing me a tiny amount in return for many free services it doesn't really bother me.