I'm not a lawyer and this is not legal advice

But...
As an individual, I don't think there are any problems with donating to any charity through any means.
The problem comes in when you try to advertise on your site that you are donating to that charity.
Since the charity has no way of knowing how many sales you have, they can't be sure that you are following your word to donate a certain amount of sales, and they don't want you profiting from their name.
Just don't use their name on your site. Use a generic statement about donations, and try to be as transparent as possible on your website in regards to number of sales, how much you have donated to date, receipts from donations made, etc.