A woman in Oklahoma bought a 3G netbook from RadioShack for $100, subsidized by a two-year data plan from AT&T Mobility. That plan comes with a 5GB monthly data cap, which she exceeded, and as a result her first monthly bill was over $5,000. Now the two companies are facing a class action lawsuit that alleges they are not clearly disclosing to purchasers that overage fees could be “astronomical.”