Earn is the one feature in this product where you're the one being paid rather than the one paying for a feature: it shows relevant third-party partner offers on your thank-you and order-status pages, after a shopper's own order is already complete, and you receive a share of the resulting revenue. Nothing about Earn touches your checkout flow, your inventory, or your fulfillment process — the offers shown are for products you don't stock and never have to fulfill, which is also why it's placed specifically on post-order surfaces rather than anywhere pre-purchase.
Activation is a program you opt into rather than a setting you toggle — it's governed by the Earn Program Participation Agreement, which covers how revenue share is calculated and paid out, what counts as invalid activity, and which advertiser categories are eligible or excluded. Worth reading that agreement directly rather than assuming Earn behaves like a standard UpsellPlus offer, since the revenue mechanics (you're paid by UpsellPlus for partner impressions/conversions, not generating your own discount-adjusted upsell revenue) are genuinely different from everything else in these docs.