Track actual shipping cost by order

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I have a situation where some of my customers are set up for monthly billing rather than per order billing. I use shopify so their individual stores can place their orders and then we invoice their head office at the end of each month for all activity during the month. I would like to be able to add in my actual shipping cost when marking an order as fulfilled so that I can report on the total cost of the order (including shipping cost) to arrange the invoice, but there doesn't appear to be a way to do this. I don't want to use the calculated rates as this may not be the exact cost of the shipment itself. Any help would be greatly appreciated!