Hi All,
I have also been trading NYC MetroCards that are about the same size as players cards and room key cards. I have found that glassine envelopes are ideal for labeling and shipping the cards in. There are many sizes and quite a few are perfect for cards. I had a bunch left over from my Stamp Collecting days.
Many times the books of stamps that you purchase out of the postal machines come in these glassine envelopes.
--Alan.