Take a shoe box and line it with either tissue paper/paper towels/bubble wrap/etc. then divide the cards up into stack that are as tall as the box is narrow. wrap each stack in saran wrap/cling wrap, to keep them from moving and to water proof them. put the stack in the box and fill any extra space with crumpled newspaper or tissue paper. make sure you tape up the box very well to keep it closed and to keep it from ripping. I usually wrap the packages I sent in plain white paper just to cover up any writing on the box and make it more presentable.
You can also do the same idea with a flat rate box, which is easier if you don't mind driving to the post office
