Amazon Auto Buy is a nationwide, buyer-side feature: a shopper sets a maximum price and a start/stop watching window on a product page, and Amazon automatically completes a one-unit, one-time purchase the instant the listing price drops to or below that threshold. It is FBA-only and excludes any listing running a coupon or promotion.
Auto Buy structurally competes with Subscribe & Save for the same reorder: because it has no promo exception, any discount a seller runs to move inventory or win a Buy Box war instead clears the Auto Buy queue at the lowest available price, pulling a would-be subscriber into a one-time sale rather than a recurring one. It also distorts seller-visible demand signals — a shopper who sets a trigger and walks away stops actively comparison-shopping, so search/demand data goes quiet on customers who are still in-market rather than genuinely churned.
Countermeasure: make Subscribe & Save the easier choice before a shopper ever reaches for Auto Buy — a subscription discount that beats waiting, a stable list price (keeping the 90-day-low trigger out of reach), and reliable FBA stock.