[E. Pluribus Unum]

Unless his website or offerings are extra special, his customers will just move to competitors rather than paying the surcharge.

Or they could click one of the provided links and download Chrome, Firefox or Opera.

Or they could just open the respective browser that they already have installed anyway.

This isn't 1995. Everybody knows they aren't limited to a single browser.

[Boogieman]

Yes, they could, and sure, everyone knows that. Still, I would refuse to patronize him simply to punish his arrogance, and I doubt I am alone. Remember, the customer is always right, even when they really are completely wrong.