Wow, that was really cool of Mark. First off, does he even have 20$ worth of stuff? I never look at the prices, but know most of his stuff is readible affordible (I think you can get all his products for under 20$ if my math's right).
Whispering Woodwind is a great module of low level characters. I ran it at the first Ohio Gameday and it was a big hit.
Potent Portals is good (and Clay did the art). It's all about magical doors, entryways, etc.