A storage system for storing product items includes a grid structure and a number of first storage bins configured to be stored in vertical stacks in the grid structure. Each first storage bin is configured to contain at least one product item. A vehicle is arranged to move horizontally at the top level of the grid structure, and further arranged to pick up, carry, and place the first storage bins at desired locations within the grid structure. The storage system further comprises a robot device comprising a movable arm with a picking mechanism in one end thereof. The robot device is configured to move a storage item between a first location and a second location by means of its picking mechanism. The first location is the location of a first storage bin stored in the storage grid.