The best post-purchase upsell works because it can't hurt your initial conversion rate. Since the customer has already paid, you've locked in the original sale. As Phillip from Future Commerce explained, this allows you to show a new offer without affecting that first transaction. It capitalizes on the excitement of the moment, what Eric Bandholz on Ecommerce Conversations calls the "high of just placing an order," to increase basket size with zero risk to the initial cart. This is the critical distinction from pre-purchase offers that can add friction.
The most effective offers are either a complementary product or a small upgrade. Nik Sharma made the point on Limited Supply that this is how you turn a one-time buyer into a more valuable customer. Don't overthink it. Lachezar Voynov mentioned on 2X eCommerce Podcast that his clients see a 10-15% AOV increase just from simple Post-Purchase Upsells. You could offer a matching accessory, a sample of a new product, or a subscription to the item they just bought.
Your first step is to analyze your order data and find your most common two-item combinations. That's your first upsell offer.
