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How do I use robots.txt to tell google to not add item to the shopping cart?

Well, theoretically, your Add To Cart button could have an href with a path that’s banned in robots.txt, but overridden with JS.

But most online stores should be happy to have Google crawling their prices and showing up under the Shopping results.

Erm... hide the shopping cart page behind robots.txt?

As someone who has seen way too many robots.txt files that's exactly how you do it.