Well, it turns out that I wasn't overbilled by $100, I was overbilled by $179.95. Apparently, because I signed up for two services at once, not only was the activation fee supposed to be waived, the two installation fees should each have been $25, not $100. Oops.
Great way to impress the customer, Comcast. At least it was resolved quickly and without argument- or, at least, I've been informed it has been; I'll find out on my next bill if they're actually appyling the credit back!