Humidity, not so much, but water is bad...very very bad. Plastic wrapping helps keep cards safe from minor incidental rain or snow damage to packaging.Wow, I appreciate the informative reply - particularly in regards to packaging the cards. Now that you mention all the possible ways that the cards can get damaged in transit... is humidity ever something that gets accounted for? Or is it too minor an issue to deal with?
Is there some sort of standardized grading scale for the condition of cards? I had seen a loose one before for a card game I used to play called Magic the Gathering, wondering if there's something similar for trading cards.
As far as grading: Yep, cards are "condition sensitive", meaning a perfect card may sell for $100 but a card bent in half is basically garbage material.
The only times severely damaged cards are still valued is when the card is highly sought after, like the baseball cards from the early 1900's or a modern "star" auto. Most autographed cards today retail for a couple of bucks at best, so if they are bent in half they could never be sold.
That said, if you're a collector and you like the card, keep it. Who knows...maybe a hundred years from now someone will see it and pay ten...twenty bucks for it. Then again, a hundred years from now twenty bucks would only get you a "penny candy" anyway so...