The most common offer you'll find on a cart summary page is "you are $X away from free shipping" when a free shipping promotion is offered on purchases above a certain dollar amount. The message is called a "carrot," to entice customers to add a little more to the cart to qualify for the offer.
A unique application of the "carrot" based on cart contents is used by Maghound. Maghound is a kind of "Netflix for magazines" -- Maghound's subscription service allows customers to build their own subscription bundles for a monthly price, with the ability to change subscriptions at any time during the year.
Maghound has 4 subscription tiers:
Customers begin selecting magazines and after adding a title to the cart, the cart summary box updates with the title, and a message (in the yellow box) reminding the customer how many titles are included at the current tier of membership.
Once you hit your first tier (3 titles), the message changes to "Pick 2 for $3.00 more per month!"
And so it goes after the second tier is met:
The cart summary continues to upsell as the cart contents change. The "carrot" depends on what's already in the cart:
Once you fill tier 3, each additional magazine is offered for only $1 more (who can resist?)
Maghound is a unique example of guided selling and promotions in the shopping cart. Another industry that typically uses guided selling in the cart is telco. Customers need to be led through the complex process of bundling a handset with a contract, services and accessories. This example is from Telus Mobility.
Telco's guided selling could incorporate targeted selling if the links in the cart summary were based on what's in the cart (for example, the link to accessories will land on a page that shows only products compatible with the device in the cart.)
Guided selling / targeted promotion implementation in the cart summary is so custom to a business that you most likely need a custom build to make it work (both of the above are Elastic Path customers). As more retailers across industries incorporate guided selling, product bundles and subscription/loyalty programs, we'll see more examples of this around the 'Net in the future.