Tools>>Letters>>Edit Client Letter>>Custom Client Letter.

 After the  Client Letter opens.... did you check (on the right side.. under Letter Paragraphs), the "Amended Return' and then save the letter.  

But you should only do this in a Custom Client Letter...or it will change all of the 'Standard" client letters going forward.

I have my default to ONLY print the Custom Client letter with returns... makes me open the custom letter for each client and review/edit it. Reviewing the custom client letter has saved me a bunch, bunch of times.