S.C.
I personally HATE having to fill out my own thank you card envelope at parties. I already gave you a gift. Now you want me to address my own thank you note? I think it's rude. But that's just me.
Why don't have you have sign in book that people can write in as they enter the party. You can have a space for their name and for their address. You can title it the "Let's Keep in Touch" book and that way, even people who do not bring presents can be thanked for coming and/or kept in touch with.
Good luck!