This issue will be down to an incorrect GL Sales Account in the GL set assigned to the Customer.
For Products and Services, confirm GL set in the Masterfile.
To confirm the GL Set assigned to the Customer
Go to Customers, Masterfiles, Customers, Enquire.
Go to the Account Details tab.
In the General Ledger Defaults section, note down the GL Set Code.
To update the default Sales Account in the GL Set
Go to Setups, General Ledger, General Ledger Sets, Maintain.
Select the GL Set Code that was assigned to the Customer in previous steps in the Set Code field.
In the Sales & Purchasing tab, update the Sales Code field to be the correct GL Account.
Update any other incorrect GL codes in the GL Set and click Accept (F9) to save the changes.
Re-try creating an invoice for that Customer adding a Sundry line and confirm that the correct GL Sales. Account now displays on the line.