We do have the little envelopes onboard, just not in every seatpocket. If you ask for an envelope and the crew member say they haven't got one/can't find one, let them know it's in the stationary drawer/bag

unfortunately, some crew don't read their work emails, so won't be very informed. Personally, I don't like walking through the cabin shaking a bag at people, but will be happy to provide an envelope and take it to base where it's posted. I just think the bag shaking makes it all look tacky. My opinion, of course.