We just launched a new app called FIFO - Order & Backorder Manager. We created this app as we also use Shopify for our Online Shop. Our problem was to identify which orders are ready to be fulfilled and which orders are still waiting for a product to be back in stock (backordered).
Based on custom "Payment Status" rules and physical, digital and reserved inventory the app will let you know which orders can be fulfilled!! Also nice, you can create picking lists for selected orders and you can print your packing slips, invoices and so on with some integrated apps like: Order Printer Pro, Orderly Printer & Order Printer.