Vintage for vintage please unless I make the proposal
I will send all cards in plain white envelope (PWE) unless agreed otherwise or if card values are high. Mailing more than 18 cards may be split into separate envelopes. I usually buy postage at the post office and get a receipt so I have some kind of verification.
Single cards will go out in a toploader. Multiple cards will go in a 9 pocket sheet, folded and placed between more rigid cardboard. The goal is to keep it thin to keep it cheap and avoid package charges.