If a sales order is received from a customer with two different items (Item A and Item B) and both need to be put on back order, Zoho Books does not allow you to part ship the sales order until all items are received.  This is inconsistent with a standard sales order process that does allow you to part ship an order, so I'm not sure why Zoho decided to change the functionality depending on whether the ordered item is in stock or needs to be backordered. I have confirmed this as the standard functionality with Zoho Customer Support (if anybody knows of a better way, please let me know).

I suggest this functionality be changed, so that regardless of whether a sales order has back ordered items or not, it can be part shipped.  For example, if item A is received Zoho Books allows you to part ship if the customer wants to deliver this item whilst waiting for Item B.

A better way entirely is to have a flag in the customer record that indicates whether a customer agrees to receive part shipments Yes/No (some do, other's don't), and this get's applied to a sales order BUT can be changed where necessary (i.e. on a once off basis).