Cross-sell on PDP
'Customers who bought this also bought X'. Natural on product pages. AOV lift 5-10%.
Upsell in cart
'Add this for $X more and get Y'. Cart page is where price comparison happens. AOV lift 10-15%.
Bundle at post-purchase
Thank-you page: 'Add a bundle for 20% off.' Removes friction (already decided to buy). 15-25% take rate.