The standard client letter should show Mail In or EFiling/EPay info (if scheduled within the program) by default.

Im not sure what kind of trigger codes are available for after filing instructions, have you tried to Edit the Client letter to see whats available?  Or you could always add your own paragraph.

If youre referring to the 2025 program, the letters may not be printing correctly until closer to the opening of the filing season.

When you are in home base view select "file" then scroll down to "print options" then choose options you want.