Religious charities rejects donations.
From time to time I hear from theists that atheists tends to donate less than theists. Studies supports this, but I think there may be an underlying issue other than their lack of belief in gods.
It is not unheard of that religious charities, such as the Salvation Army rejects donations if they think they go against their religious belief. With one particular case with the Salvation Army, they rejected toys of Harry Potter and Twillight, because it promoted the occult and witchcraft.
They also reject money donations from groups that they didn’t like or if the money was attained in such a manner that they did not like. Such as money won in legal gambling or lottery. Also money from musicians or likewise that does not share their religious belief.
I have focused a lot on the Salvation Army, but I would speculate that a non-believer would feel uneasy to donate money to charities that are religious, which the majority of charities are.
So what we need is more focus on secular, and open charities, to make sure we know what the money is spent on.