[cstreit]

Complimentary as in related.

If your main objective is to sell product X, but it is often purchased with products Y or Z, you should carry those and make it easy to buy at the same time. Related items being displayed on the same page.

If you and your competitors are similar in price then the consumer also knows this. Those consumers that want to buy the package of X, Y, and Z are going to skip you and buy from somewhere else so they don't have to make two purchases and pay shipping twice, again assuming prices are relatively equal.