The difference of a fixed percentage on a variable cost is very minor. If you're recommending a $100 monitor with a 4% affiliate fee, that's $4. A $150 monitor is $6. Making good recommendations (i.e. stuff people want to buy) will easily outweigh the payment difference.
Still, it's important to have a policy about it and be transparent - which Wirecutter (and the NYTimes surely) has.