In order to sync upsells back to Guestline, you must locate the Product Code ID in Guestline and add it back to the Billing Identifier section of the Upsell
Go to Settings > Products
Choose any upsell/products from the list or create a new one ➝
Once you select the upsell you can set the setup for it ➝
The Billing identifier is - "Product Code"
Copy the Product Code into the Upsell. Go to Advanced and paste the Guestline Product Code ID into the Billing Identifier section
Save your changes and then you're done!