9. A computer-readable storage medium having a computer-executable user interface component for presenting to a user items complementary to an item of interest, the computer-executable user interface component comprising: one or more display elements from which the user can select a combination of items as a unit, wherein the combination of items includes a user selected item of interest and one or more items that are complementary to the user selected item of interest, each of the one or more complementary items being automatically identified based on rules data that define which items are complementary to the item of interest based at least in part on prior sales performance of the complementary items, and wherein the user selected item of interest and the one or more automatically identified complementary items within the combination are displayed as a unit to the user as pre-selected for purchase together as a unit; and one or more selectors from which the user selects, from a list of substitute items that are automatically identified based on the rules data, a substitute item complementary to the user selected item of interest, to replace a complementary item in the combination of items displayed as a unit, the list of substitute items displayed in the user interface component upon user interaction with one of the one or more selectors; and a purchase control that enables the user to initiate a purchase of the combination of items as a unit.
9. A computer-readable storage medium having a computer-executable user interface component for presenting to a user items complementary to an item of interest, the computer-executable user interface component comprising: one or more display elements from which the user can select a combination of items as a unit, wherein the combination of items includes a user selected item of interest and one or more items that are complementary to the user selected item of interest, each of the one or more complementary items being automatically identified based on rules data that define which items are complementary to the item of interest based at least in part on prior sales performance of the complementary items, and wherein the user selected item of interest and the one or more automatically identified complementary items within the combination are displayed as a unit to the user as pre-selected for purchase together as a unit; and one or more selectors from which the user selects, from a list of substitute items that are automatically identified based on the rules data, a substitute item complementary to the user selected item of interest, to replace a complementary item in the combination of items displayed as a unit, the list of substitute items displayed in the user interface component upon user interaction with one of the one or more selectors; and a purchase control that enables the user to initiate a purchase of the combination of items as a unit. 10. The computer-readable medium of claim 9 , wherein the user interface component further comprises: a price indicator indicating a price of the combination of items as a unit, wherein the price indicator is displayed with the combination of items in the user interface component and is automatically updated as displayed upon selection of the substitute item.