There's tons; Wikibuy, Honey, etc, etc. I use Honey but I've used a bunch and the results are generally the same. The paranoia inflicted upon me by HN crowd saying I'm overpaying on Amazon by 30% makes me investigate all these claims and I just find them all to be completely false in my case.

I'm not saying people don't have these experiences, but I don't. And even if I did for like 5% or something, the shopping experience on Amazon is 10x better than Walmart.com or Target.com (where it's more like 100x), and that isn't worth nothing. It's like patio11 insisting that basis points fees aren't worth arguing over on Stripe when you are infinitely more productive using Stripe's developer kit vs. Authorize.net or any other terrible gateway, and people everywhere on HN claim how expensive Stripe is and how you can save money and don't factor in even a very conservative estimate on labor hours for a C-level exec or a manager in a company.

It doesn't take much time saved or annoyances avoided for a high-level executive to make a small price difference be completely irrelevant. And that's what Amazon has mostly done.