I do this occasionally – basically you sign up to a service and every few days they email you a new offer. If it’s something you fancy you email back, get a discount code and “buy” the item as normal. Most of the time the item is free or literally pennies and if you’ve got Prime you very rarely pay for postage.
Sometimes there’s items offered you’ve no interest in so even for free you don’t bother, other times there’s been some good stuff.
How truthful these reviews are I wouldn’t like to say, I’ve left some which have been fairly scathing and still get offers of new products but it’s easy to see how people would try and keep things sweet and keep the free kit incoming.
Edit: Just checked the last email I had from the site I’m registered with, they’ve got a new boardgame, some car wash concentrate and a car paint chip repair kit for the vehicle of my choice. RRP of these is £15, £13 and £40 respectively.
Now I’ve seen it again I might get the chip repair kit!