Would you like fries with that order?
Imagine you are at a Big Box store in the electronics department and you overhear a salesperson talking to a customer,
"That's a great flat-screen TV. Shall I add an extended 3 year service contract for $149 ?"
That is the classic example of an up-sell, introduced as you purchase new electronics.
Instead buyers often agree to a lower-priced offer, such as a 1-year service contract for only $39, technically a down-sell. However, it costs the seller almost nothing and provides a large profit margin. Most electronics failures occur within the normal warranty period, which are covered by the manufacturer. (Kushner, 2012)
Some items are frequently bought together. Just stop by the baking section of your grocery store and see Karo syrup near the pecans – two ingredients in pecan pie (with the recipe prominently displayed on the bottle). Shop for caulk at the hardware store and you'll find caulk guns nearby. This is cross-selling. But cross-selling on-line is more complicated. For example, if a customer buys a plaid shirt, they might also be interested in khaki slacks. Stock related inventory in the same fulfillment center. You'll save on shipping and picking costs, important issues for customers (see Reducing Shopping Cart Abandonment).
These techniques work on-line as well as in person. We've all seen them. Product details include package deals; buy this item and a related item for a special price. Buy a printer and see an offer to buy ink cartridges.
A newer concept is up-shipping. During the gift-giving season people are more likely to select faster shipping. Amazon, eBay and WalMart are experimenting with same day delivery, though not everyone in New York City or San Francisco can afford to spend an extra $10 for this service. You may have seen some catalogs that say, "Order by December 22nd and receive your gifts by Christmas. Guaranteed." That requires expedited shipping, which comes with higher margins. Let your customers know that this is available.
However, all these techniques only encourage the shopper to put items in their virtual cart. Checking out is often terminated early when shoppers abandon their carts.
Kushner, Aviya (2012) Consumer service plans: Worth the cost? http://www.bankrate.com/brm/news/advice/customer-service/20050815a1.asp