In most cases, Cashback applies to an entire shopping cart rather than just a single item. So, for instance: if you create and share a SplitLink to a pair of glasses at WarbyParker.com – but your friend ends up buying a different pair of glasses as well as an additional accessory -- the Cashback amount will likely take into account ALL items purchased (not just the one pair of glasses that you specifically recommended).
Note: Cashback amounts/percentages vary by retailer and in some cases, even by product. (In fact, a few retailers exclude certain categories from Cashback-eligibility altogether.) So, it's possible that in some cases certain orders within a shopping cart might not produce any Cashback at all. For more info, see this article on Qualified Purchases.