Support of Store Pickup for Multi Source Inventory
Customer place the order and wait to receive a "Ready for pickup" e-mail. (This is a separate e-mail from the order confirmation.) Bring his ID, credit card, and order number to the store and pick up the ordered items.
At the time when the order is being placed on the "Shipping" step we introduce an ability to choose "Store Pickup" out of allowed shipping methods.
We apply an assumption that each order should be fulfilled just from the only pickup location.
Note: if some need to specify different pickup locations per cart item(-s), internally that could lead to functionality similar to Multi-Shipping when several orders created, for each corresponding pickup location