You inspect it more closely, It's made of a sturdy, matte cardstock, while the companion envelope is a perfectly suitable paper.
A quick count of the 5" x 7" greeting cards shows that each pack contains 3 cards.
This particular item has a card displayed nearby. You pick it up and read. 'Dull cardstock can be signed with permanent marker or ballpoint pen. Empty on the inside for whatever personal touch you want to add!'