The Fulfillment Order documentation states that the delivery_method/method_type can have a value of "pick_up".
However, in the API, when fetching the Fulfillment Order object, a pickup is actually marked as "pick-up" (hyphen vs. underscore).
Result from API:
The value in the REST API is actually unexpected. Typically, string values like that use underscores and not hyphens.
Thanks for flagging this - I've opened up an issue with our internal teams to either change the docs or the value in the API.