Adding Shipping Costs to purchase orders after booking in

Adding Shipping Costs to purchase orders after booking in

Hi

I need some help with adding shipping charges which our suppliers bill us for sending goods to our warehouse.  These charges vary depending on the weight and size of what's being shipped so we never know how much this will be until we receive the invoice.  That's not too bad as we've created an item (service) that goes to cost of sales that we can add to a purchase order after we get the invoice - a bit messy and if anyone knows a better way I'd be grateful to hear it.  However, we also use the back order function to order and sell some of our products and this doesn't allow any items to be added that were not entered at the time of creating the order.  We have a work around whichis creating a separate bill just for the shipping charge but again this is messy.

Does anyone have any suggestions please?