Comments
The idea that all documents for a customer should be processed the same way is a far to simplifeid way of thinking. So make it structure so that you can choose a default scenario that can be overruled for relevant documents. Especially Shipment is more or less always handled differently from other customer documents.
Option to recognize the withholding tax payable GL entry at the time of invoicing should be available. It is required in the Philippines to record and deduct the withholding taxes when the purchases become payable. In addition to this, option to make necessary adjustments for the posted withholding tax during invoicing should be available. In this way, the user who will create the actual payment can review the posted withholding tax and adjust if needed.