In addition to filing a complaint with the FCC, one may also file an antitrust complaint. Data caps with high overage fees for an essential service in areas with no competition could reasonably be seen as an antitrust violation. There is a 2013 law against retaliation. Other avenues would be contacting a congressperson or the Better Business Bureau. Single complaints won't have much impact, but a high volume of complaints did have an impact on the TWC merger attempt and the recent net neutrality ruling.
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