Ahh, Memphis beat me to the punch....
You're probably right, Memphis: Serpentor was first offered in stores. And then maybe later, he was offered as a mail-in figure.
And don't forget about the Cobra vehicles too! I picked up the HISS and FANG (the tank and the mini-helicopter) back in the day thanks to those mail-ins.
Ridiculous tangent: How many points do you think they would've required to get the USS Flagg (if they ever offered it)?
And speaking of which: How many points was the ol' Flagg worth?