Acme Watches spends $10,000 on each of two campaigns. Campaign A sells $30,000 of discounted watches with $9,000 gross profit: ROAS 3.0, POAS 0.9. Campaign B sells $24,000 of full-price watches with $13,200 gross profit: ROAS 2.4, POAS 1.32. By ROAS, A wins. By profit, B does, and A is losing money.

ROAS treats a dollar of revenue the same whatever its margin. When products, discounts and bundles carry different margins, ROAS can steer budget toward the least profitable sales. POAS steers it toward the campaigns and creatives that make money, at the cost of needing product cost data per order.

Load cost of goods into your store or reporting so profit per order is known, and define gross profit the same way every time, with or without shipping and fees. Then set your target as a POAS, and point creative at higher-margin products and full-price offers.