Well, I guess you'd have to consult a tax attorney or contact the IRS (assuming USA) or equivalent where you live.
Honestly though, is the amount even consequential enough to bother with? I guess it potentially could be if you want to spend a lot but in that case, why not just give to the charity directly?
I think it is nice Humble Bundles do this but I'd never even consider the tax consequences. I just wouldn't bother myself.