the 20 slot storage chest is not available in the shop for diamonds it is for rubies only.
this item should be available for diamonds all the time. people need storage and to have these in the shop for rubies only is not a good decision at all. this item used to be in the shop for 99 diamonds and players bought them some had close to 100 of them in their houses. and then this in turn made them buy house energy to keep all of them open.