I've just set up a store with custom order fulfillment to one fulfillment service, I've used my email address to test (have tried two, actually). I've followed the instructions on the shopify help pages- products are set to be fulfilled from the fulfillment service, the email is correct but when I manually "fulfill" the order, unticking the "notify customer" checkbox so only the packing service is notified nothing happens. No email is received.
Does anyone know how to resolve this?
It turned out that you have to assign the fulfilment method to the actual products, and if you've had an order before you make that change no amount of re-sending the fulfilment request will work. It worked for the next order though.