I doubt that there are many (if any) using the USB Wifi option, Bill - all the recent laptops have the Wifi card installed by default, and most with the older ones where it was optional had them installed originally too. Usually if the Airport card dies, another one will work - so your situation is fairly rare. And unless you actually try the USB Wifi connector on your Powerbook, you won't know if it works or not since the actual reason for the Airport cards not working hasn't been established.
I wouldn't worry about ordering from AftertheMac:
QUOTE
AftertheMac.com provides a 6 months no questions asked 100% money back on any return if you are unsatisfied in any way.
We also provide a minimum 1 year replacement warranty on all of our products.
Pretty low risk if you ask me.