We have a few customers who have a situation where their techs will go on a supply run. During this process in their old systems, they have provided a blank PO to the supply house (basically just a number) and then they will be able to match it to the invoice that is later sent after the tech picks up his materials.  

My solution is to mark for PO on the mobile app (using a nonstock item) and generate the PO. This will allow them to have a PO number that they can provide to the supply house when checking out so they can later match that invoice when it is sent at a later time (most likely 1 business day). The tech will then go into that PO he generated and update the cost. The cost will then flow back to the appointment when the PO Receipt is released. However, since the tech is paid on completion, he will have to manually add up the amount to take the prepayment rather than the system calculating it. 

The point of using a PO with a nonstock here is to create a check to make sure that the correct amount is applied to this appointment. This way the AP department will be the one confirming the amount on the receipt to make sure it is accurately recorded by the tech. 

Are you handling this same situation differently?