Suncoast/Sam Goody/Musicland has made an error. They're listing the retail for the Firefly set as about $29.99. Which means, with their pre-order discount, you can get the set for $19.99.
No, that isn't a typo, and, yes, it has been confirmed. By many people, including myself, all over the US. $5 will hold it for you, if you're super strapped for cash right now. And it won't matter if they discover their mistake at a later date. The price you pre-order at is the price you pay. Apparently, they make errors like that occasionally.
So, if you're up to a trip to the mall? Get your DVDs cheap!