A method of operating an order processing system which comprises a goods storage and retrieval system, the goods storage and retrieval system comprising: (a) an ordered disposition of containers in a grid of storage locations defined by a structural framework, such that containers are disposed vertically in layers, and in multiple columns and rows, (b) a plurality of robotic load handlers for retrieving containers from any one of the storage locations; and (c) at least one rail arranged above the grid to enable access to each of the storage locations by at least one of the plurality of robotic load handlers; the method comprising: a robotic load handler storing, in the goods storage and retrieval system, a storage container containing one or more stored items; a robotic load handler retrieving a storage container from one of the storage locations, the storage container (10, 70) containing one or more items for fulfilling a customer order, and transporting the retrieved storage container containing such items to an order picking station; wherein the method comprises: storing, via at least one robotic load handler: a first set of storage containers (10, 70) configured for storage of items within the grid, and a second set of delivery containers (10, 80) configured for delivery of the items to customers who have ordered them, the delivery containers of the second set being configured to be contained within the storage containers of the first set, at least some of the storage containers so stored in the grid being in the form of a container combination formed by a storage container of the first set containing a delivery container of the second set. (a) an ordered disposition of containers in a grid of storage locations defined by a structural framework, such that containers are disposed vertically in layers, and in multiple columns and rows, (b) a plurality of robotic load handlers for retrieving containers from any one of the storage locations; and (c) at least one rail arranged above the grid to enable access to each of the storage locations by at least one of the plurality of robotic load handlers; a robotic load handler storing, in the goods storage and retrieval system, a storage container containing one or more stored items; a robotic load handler retrieving a storage container from one of the storage locations, the storage container (10, 70) containing one or more items for fulfilling a customer order, and transporting the retrieved storage container containing such items to an order picking station; wherein the method comprises: storing, via at least one robotic load handler: a first set of storage containers (10, 70) configured for storage of items within the grid, and a second set of delivery containers (10, 80) configured for delivery of the items to customers who have ordered them, the delivery containers of the second set being configured to be contained within the storage containers of the first set, at least some of the storage containers so stored in the grid being in the form of a container combination formed by a storage container of the first set containing a delivery container of the second set. a first set of storage containers (10, 70) configured for storage of items within the grid, and a second set of delivery containers (10, 80) configured for delivery of the items to customers who have ordered them, the delivery containers of the second set being configured to be contained within the storage containers of the first set, at least some of the storage containers so stored in the grid being in the form of a container combination formed by a storage container of the first set containing a delivery container of the second set.