If the 6x6 is rigid due to a hard plastic sleeve, it will probably incur the nonmachineable charge of 85 cents. I use one of those cheap little digital scales to weigh before sending.
Index cards is a good idea. I had been using 2 promotional postcards (thrown away by the box on the street here in NYC by restaurants, spas, etc when the promotion ends) as protection in an envelope. I ran out and with everything being closed there aren't any promotional cards being thrown away. There aren't even any event postcards being distributed around.
the sleeved card between the 2 in an envelope only required a 1st class stamp. If I put more than 2 cards in, then it goes up to 85 cents. It seemed to work well.
I mailed a small box today, the box was a clean but used box for some kind of organic crackers, the name printed on the side, the clerk got all upset at the company branding and made a big deal out of putting stickers over everything. (The lady next to me was sending a box that had Playskool printed on it and that was accepted without comment).
