CEO of Cover here. This is super surprising, as we sell our own policies + own a national insurance brokerage. No need to share/sell your information at all (nor would we, as a matter of principle), because we have access to most carriers in the US. We'd prefer to sell you a Cover policy or one from our partner panel that can be managed via our service.
Do you mind sharing which companies sent you physical mail? I'm at karn at cover.com. Would be great to dig in.