Custom line items can be imported with the sale channel's line SKU. This works great for Shopify custom orders as well as stores with plugins that automatically delete listings after the order is placed.
This setting is enabled by default on all new stores created after August 2023. To opt-in your existing store's:
Go to your store settings
Open the Catalog setup settings
Change the selection under "Custom line item policy" to "Use line SKU if possible"
Items will be built or linked automatically based on your listing import setting. If you were directly invited to use this feature previously you may need to re-opt-in your stores. Supported initially by Shopify and BigCommerce, available on other channels by request.