Postcards have three dimensions, length, width and thickness. Although the thickness is very fine it must still hold up to the rigors of the Postal Service’s sorting machinery. Paper weights (thickness) are expressed in points and 1 point equals .001 inch. So 10 points = .01” . The minimum stock commonly we would ever recommend is .011” (also called “100# cover”). You may also want to consider something even heavier, 14 pts.
Paper stock is available in either gloss or matte finish. Contrary to popular belief, a matte finish actually has a slight sheen to it. Three additional finishes often used to protect the card and keep it sharp looking are Varnish, AQ (Aqueous) and UV (Ultra Violet). AQ and UV are now applied more than varnish and the short answer to the difference between the two is that AQ has a softer shine while UV offers a high gloss finish.
Hint #1: It is nearly impossible to address postcards that have been pre-finished with AQ or UV. Even address labels won’t adhere. Design your postcards either with no coatings on the addressed side or with a “mask” of non-coating on the address panel