The Cross-sell feature allows you to offer an additional product to buyers when they add an item to their cart.
Up-Sell: Replace the item they added to the cart with another item
Cross-Sell: (new!) Add an item in addition to the item they added to the cart.
Cross-sells are configured per-product on the product Edit pages just like up-sells are. You can specify the price point used, so you can use the feature to offer a special one-time discount when adding a product to the cart. For example, the regular price might be $24.95 but you can create a $19.95 price point and use it for the cross-sell on a complimentary product.
Please be aware though- It is possible to chain products together with Cross-sells and up-sells to make an annoying endless list of this-or-that choices when adding items to the cart. We recommend avoiding that.