As of API version 2025-07, fulfillment service apps can use the new estimatedShippedAt argument to specify when they estimate the fulfillment order to become fulfilled. Shopify Fulfillment Network partners will be expected to provide this value when accepting Fulfillment Requests.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/new-estimatedshippedat-argument-added-to-fulfillmentorderacceptfulfillmentrequest-mutation