"I have a question, when sending a letter to an actor or show asking for signed playbill, does anyone send cash to pay for the return shipping? Or is it not required? "
Um, you should always include a self-addressed and stamped envelope if you are requesting a Playbill. Not including that is pretty discourteous. I mean, even sending cash means that an actor or stage manager is going to have to go out of his or her way to buy stamps and perhaps proper envelopes to send a Playbill.
I think, perhaps, the only time it is acceptable to send cash instead is for people who are writing from out side the United States who do not have a way to buy U.S. international stamps.