Shopify follows a set of rules to determine which location an upsell order is shipped from. Stores with two or more active locations should have access to order routing, which can be found in the Shopify admin underDocumentation Index
Fetch the complete documentation index at: https://docs.aftersell.com/llms.txt
Use this file to discover all available pages before exploring further.
Shipping & Delivery > Order Routing. If you don’t see this option, ensure you have at least two active locations. If you do, and the section is still missing, your store might be using the legacy Location Priority system, which is located under Locations > Location Priority.
Shopify applies these default rules for selecting a location for fulfillment as of the time of writing this article. If desired, an additional rule can be added to prioritize specific locations.

Customizing Inventory Locations (Advanced)
In Aftersell, you can select which inventory locations are used for 1-click upsells. Please note that this is an advanced setting, which requires assistance from our support team to enable.- Once added by our support team, you can find these inventory settings by navigating to Aftersell > Settings > Inventory.

Note: Automatic Upsells use Shopify’s Recommendation APIs, which don’t allow filtering by inventory location. Aftersell checks stock by location afterward, but the initial recommendations from Shopify can’t be limited to a specific location.
You can read Shopify’s documentation on this here