Return labels (personal, not business)

I'm trying to send a check to my mortgage so they can sign it and return it (insurance check). They said it goes fastest if I include a self-addressed/paid envelope.

I'm going the flat-rate route. But how do I print a return label when I don't know the date they'll send it back? Do I just guess? Am I missing something in how I'm supposed to generate this label and/or include return postage?