In order to sync Payment for Stay and upsells back to Stayntouch, you must locate the Charge Code ID in Stayntouch and add it back to the Billing Identifier section of the Upsell. If there is no existing PMS Connect ID for the service, please create a new one.
Please note: only a person with Admin user rights in Stayntouch can access the Charge Code ID.
Go to Configuration > Integrations > Utilities
PMS Connect ID Lookup
Filter the Entity for Charge Code and then find the Charge Code ID
Copy the Charge Code into the Upsell. Go to Advanced and paste the Stayntouch Charge Code ID into the Billing Identifier section
Save your changes and then you're done!