Suggestions:
If there is a confirmed Sales order with 2 items. Then users need to ensure that if the order is used in manual despatch. If the Sales order is used in despatch , made manually of having first item code, using some quantity. Or if the system takes available quantity because the ‘Available” flag is active. Then the available stock was despatch of the complete quantity.
Therefore, when the same order is used in the post order process then the second remaining item will only be visible which is not used in manual despatch. And it can be posted. Users can, if required, select Club pending order, then all the pending orders irrespective of the series will despatched under one single despatch id.