API Support for "Store Pickup"
Have you attempted to use the Fulfillment API to mark an order as ready for store pickup?
Turns out you can't - at least, not yet.
According to Shopify this feature is coming to the API but it's available yet.
In the meantime Ben Farhner has come up with workaround by having merchants add custom shipping rates for local pickup.
Check out the forum discussion for yourself