Purchase Order Prices on Commodities (Precious Metals)

 Our purchasing department has a problem in that the price of what they are ordering (precious metal) is not known until it is shipped.  Like a currancy translation in banking, the price (exchange rate) is locked at the last minute.  In order to throw a correct PPV and state the liability correctly we have to go back in and update the P.O. price after the product ships.  This is very labor intensive.  Any suggestions on how to make life easier for purchasing?

Thanks!