What? I would forgo the donation, and wear jeans anyway. For me it smells of coercion to fund something.
400 * 15 * 12 = 72,000 / yr that your employer may be deducting from wages on the backs of their employees... voluntary or not.
Then they get to advertise their generosity and optional dress code.
I hate thinking pessimistically like this, but you get conned enough times... and it's not like they actually need a dress code, if they're comfortable with taking money for people not abiding by it.