It's a crap shoot, but I will usually put 4 forever stamps on for a single card. Even it's it's a thick one, it usually works. Once you get over a few cards though, you've gotta bite the bullet and ship as a parcel. It's BS, but it is what it is.
I'm also open to hearing other suggestions as well.
#1 Collection : Barrasso
Other : Mario Lemieux, Pens cards that look good
NEED: Barrasso ITGU Nameplates and Crosby National Treasures Base Parallels