XU has exactly that. It has the "rationing pipe" which keeps exactly one stack of an item in connected inventories and the "ultra rationing pipe" which is the same except for it does a single item instead of a stack.If you had multiple items in the same chest, you could attach for example 5x itemducts to the chest each with their item filtered. The redstone signal that calls for the cart to be spawned would also then extract the specific item by applying the redstone signal to the proper itemduct. Just not sure how to control that you only get a single item out with itemducts. Maybe there is some other mean that has finer control.