Just as inducing self-selection by offering a menu of versions enhances the monopolist's ability to extract surplus, so can selling different products as a combination package. Two such techniques are bundling and tying. The practice of bundling consists in selling two or more products in a single package (bundling is said to be ‘pure’ when only the package is available, or ‘mixed’ when the products are also available separately). The distinguishing feature of bundling is that the bundled goods are always combined in fixed proportions. In contrast, the related practice of tying (or tie-in sales) is less restrictive in that proportions might vary in the mix of goods.

Economists have given different explanations for bundling and tying. First, some explanations are too transparent to merit formal treatment. In the case of perfectly complementary products, such as matching right and left shoes, no one questions the rationale of bundling: there is virtually no demand for separate products and bundling them together presumably conserves packaging and inventory costs. In other cases where products are not necessarily complements, various cost efficiencies might provide a basis for profitable bundling. In many instances the opportunity cost for consumers to combine various components typically exceeds the assembling cost of the manufacturer (take, e.g., a personal computer).