Hope you to include 2 modules that's Purchase order and Delivery Order. Recently we purchase project management and start to use it. With the first project we started now, we found it missing 2 feature. That is purchase order and delivery order. Hope WHMCS team will consider these 2 module. I guess Delivery order will be much more simpler as it just same as invoice but change the title to Delivery order and remove the price.
For Purchase order I am not sure but I guess just same like invoice title and have another set of price list.