Generally my guideline is:
- Always ask for the PO before the service.
- With some customers they take time and it may be urgent. So there we can help one time without PO, but I want the guarantee in writing that they pay promptly after the Service.
- If this then does not happen, then we only go after we have received a PO.
- If customers pay late (more than 30 days after the invoice due date) despite a valid PO before the Service, then we will ask for payment in advance.