You can always collect when you have an open order for which you have already paid shipping - e.g. a pre-order.
This means that you can only add further orders to save shipping, but you cannot collect them with the first or a new order. This is currently not feasible from an organizational point of view.
Should an order be placed with the collect option, I will charge additional shipping costs and send the package within the delivery time.
The collect option can simply be selected as the shipping method at checkout.
Don't forget: Please always state previous order numbers in the note field.