ACP defines how a shopping agent finds products (via structured merchant feeds), shortlists them against a buyer's constraints, and completes checkout — either through delegated payment on Stripe rails or by handing off to the merchant's own checkout.
Under delegated checkout the merchant stays the seller of record: you process the order, own fulfillment and own the post-purchase relationship. The agent is a new demand channel, similar to how a marketplace refers buyers.
Shopify merchants are largely auto-enrolled. Others should keep a clean structured product feed, ship schema.org Product/Offer markup, and allow OAI-SearchBot in robots.txt.