An order processing system includes an order structure in which an order includes multiple objects representing items being purchased, shipping methods, payment parameters, and other parts of an order. The order can include one or more of each of the types of objects. Items in the order can be divided among shipping methods and payment mechanisms using a relationship model to describe the relationships between the different types of objects. Shipping methods and payment mechanisms also can be related. The order may be represented as an order object, which represents the order and can be related to particular shipping methods and payment mechanisms. Relationship objects describe the relationships between objects. Relationship objects can use different relationship types to characterize the extent of the specified relationship.