clear envelopes are .20 cents extra because they are non-machineable.
( and if your cards are square but, flat that is also considered "over sized")
and that costs an extra .20 cents BUT, if they do not fit through the slot of the plastic thing at the po then they will also be considered parcels and be prepared with some smelling salts when they get to your grand total.
(that averages about 1.95 per card in postage...)
so far this year I have only had to leave my delivery lady .20 cents for one square but perfectly flat card that has arrived postage due.