Commodity bundling is studied in an environment where the dispersion of valuations unambiguously decreases when two or more goods are sold as a bundle only. Bundling is more likely to dominate separately selling the goods if marginal costs are low relative to the average valuation, or if the distribution of valuations is very peaked around the mean.
Fang, Hanming and Norman, Peter, "To Bundle or Not to Bundle" (2003). Cowles Foundation Discussion Papers. 1715.