Upsell
An upsell is an offer that moves a shopper from the item they are considering to a higher-value version of the same thing: a larger size, a premium tier, a longer subscription, or a model with more features, so the order is worth more without selling them something unrelated.
Upselling works best when the better option genuinely solves the shopper better, and when it appears at the moment of decision: on the product page, in the cart, or at checkout. A clear side-by-side that shows what the higher tier adds tends to lift order value, because the shopper was already committed to buying and only needs a reason to step up.
The nuance is that an upsell justifies itself with evidence, not pressure. Reviews on the higher-tier product do most of the convincing: a shopper weighing the premium option leans on what other buyers said about whether it was worth the extra spend. Without that proof the step up feels like a markup, and an upsell that reads as pushy or that buries the cheaper choice costs trust and can drag conversion down.
Keep upsells distinct from cross-sells, which add a different, complementary product rather than upgrading the one in hand. Measure an upsell by its effect on average order value and conversion rate together, since an aggressive prompt can raise one while quietly eroding the other.