At Copa they're kinda famous for bumping people and making them stay in Panama City until the next flight. Of the times I've been on Copa, 50% of the flights were oversold, but I've taken none of their dirty vouchers... As thescandalousvato said, they're difficult to redeem and "taxes" on copa may be half of your total ticket, so not really that much of a deal.
That being said, I'm not a fan of Copa.. I find their seating uncomfortable, so I don't fly them unless is absolutely necessary.
You're flying on Christmas week, so if you need to be somewhere by christmas, don'tjust hope the last flight before will have a seat for you.. People in Panama said you'll have less of a priority to board on another full flight if you were bumped once.