Another totally legal approach however is to use group headers for the letter head data. If you cannot change the data binding to pass the "master" data as variables, this might even be your only chance. This worked pretty well in LL before as long as you didn't need a page count for your invoices. There was no way to reset the page count in a group header and no way to get a separate total page count for each invoice (as LL handled the whole merge print as one single job).
In List & Label 22 you now have a new property to decide if the page count should be reset and if you need a separate total page count as well. The UI is simple to handle – the "Pagebreak Before" property has a sub property that allows you to activate the new feature. This works for both group headers and report container sub items.