Sorry to resurrect a thread from the dead, but I had a massive brainwave with these.
Bought some of that clear sticky back plastic film used to cover books and things, £1 a roll from Tesco.
Cut it into squares the right size for my plastic red and yellow cards and hey presto - works a treat!
Write on with a Sharpie and it works a treat, can either pull off and replace or rub with alcohol (nail varnish remover etc) and its good to go again.
One roll did about 60 squares the perfect size for my cards, took about half an hour while watching TV and I have about 12 months worth.
Used today in my game (5 cautions), no smudging or anything, perfect. By the time you finished writing and shown the card the ink is dry and back in the pocket she goes!