Taking inspiration from the easter egg thing, why not enhance your shopping cart applications to add a set of promotional cues that store owners could use from time to time.
Basic workflow is from an item or category page, add a promotion.
Choose the type of promotional "embed" you want.
* It could be page based
-Things like randomly placed/sized/alpha icons.
-Things like dialogs presented.
* It could be element based
-Things like taking a certain action triggers the promo link/image.
If element based, choose the element that will trigger the promo. Choose the action that needs to happen on that element to trigger (click, mouse over, drag, drop, etc).
Choose the action that is taken.
* Open a page, define page.
* Present dialog with promo code, define code.
etc..
Then you can sell "promo kits" that enable different types of promotions and include everything you need to make them happen. Icon set, promo page template, small guide how to implement.
"Because its just that way."