You can make client letters print with the return or organizer:

Open a return / organizer or whatever.

Select Tools > Letters > Print with return setup

Click on Global Print options in the upper left > then "Control which letters print"

Select the return type or organizer, then select which letter and copy you want it to print with.

I thought about setting organizer to not print any pages and see if the letters would just print if I selected nothing else, but that was too much work for me. But the above is how you get a letter to print with all returns or all organizers.