In an airport checkout story from my notes, I went to buy a drink and was offered gum for the flight. The suggestion made sense in the context of the trip. A second offer on the quantity followed. What stayed with me was the relevance of the sequence.
Begin with the purchase occasion
The next useful item depends on what the customer is trying to do. Someone buying a tool may need a compatible attachment. Someone booking a service may need help preparing the inputs. The opportunity comes from understanding the task around the original purchase.
List the questions customers ask immediately after buying. Those questions are a better starting point than the inventory the business wants to clear. A complementary offer should remove a real obstacle or add a benefit the buyer can evaluate.
Keep the original purchase complete
An upsell should not reveal that the first offer was missing something essential. If an unavoidable component is required to use the product, explain the complete cost before the initial decision.
Make declining simple. A relevant suggestion can be useful even when the person says no. Repeated prompts after a clear decline turn a helpful interaction into friction.
Watch the value after checkout
Average order value is only one measure. Look at whether the extra item gets used, whether customers ask for refunds and whether the bundle creates support work. A larger transaction is not automatically a better experience.
For services, describe the difference between an upgrade, an adjacent service and a smaller alternative. Each should answer a distinct need. The useful sales question is what makes sense next for this customer, in this situation.