Adding to a delivery order

All our rentals are delivery only. Our customers often rent items months in advance and later want to add another product to their order or sometimes remove one. It is frustrating enough that a customer cannot amend their own order, but extremely frustrating that we cannot even add or remove items from an order already created even if the date is well into the future. I am told that this, pretty basic feature is on your roadmap. Can you please indicate when? Thanks.