Hi,
Well, USD20 seems a bit too much for "unauthorized" replacement wipers. I spent last winter with a set of noisy, but otherwise excellent units from a hard discount store - the pair was THREE euros, and in spite of no air spoilers, they showed no tendency to lift off, even at 110 mph+.
Currently, my W163 is sporting W164-style wipers from Valeo (see the galleries when they get back online). excellent quality, and at 45 euros the pair not more expensive than having the OEM.
I'm certain that there is already cheaper OEM-replacement wipers on the market. After all, MB is late in adapting this wiper style.
Cheers,
Birger