I think your best bet here will be to hook into the order update and check the order status or balance. I'd recommend having an order status that you move orders to (either manually or through a workflow configuration) when it's time for them to move to fulfillment. This may even just be the "Completed" status. On an update, when the order moves to this status, if it hasn't already been pushed out to the fulfillment service then go ahead and do it.