Cross sells are always dangerous. 
Ron Levy, Scott Phillips, and Dean Shannon all did it in the past and made serious bank although not as up front as a check box. 
Conversion and rentention - not to mention chargebacks and refunds.... what is the difference with checked vs. unchecked. 
I'm a firm believer in "READ THE SCREEN" if you don't you get what you deserve, but then I have an old school Aussie outlook 
