There is a software I used in the past (Duoplane) which has an amazing function for keeping track of sales order shipping dates.

It allows you to set a promise date (Estimated shipping date) per PO that is associated with an order, so that if you are not shipping from currently on hand inventory you can note an ETA before the order ships out. In the event you have items arriving from two separate locations, you can track them individually allowing you a more thorough overview of the order status.

I think this would be extremely helpful in Zoho Books/Inventory. We already have estimated ship date existing.