In addition to payment processing fees, there is some small accounting effort and liability risks for each donation. To see how this is true, imagine that 10,00

In addition to payment processing fees, there is some small accounting effort and liability risks for each donation. To see how this is true, imagine that 10,000 people donate $1 to a cause per year. (This is purposely a huge overestimate for an organization like arXiv, who I'd guess will only attract 50-200 donors.) It will then be likely that 0.1-1% (10-100) of people will cause some sort of trouble due to the donation such as - creating complications on social media because they claim they have some stake in your project - requesting custom legal documents to prove that their donation is tax-refundable (ridiculous for such a small amount, but people still do it, especially if the donation comes from a bureaucracy machine) - asking for refunds - arguing when you say the policy disallows refunds - considering your organization a paid service and asking for special technical support - thinking of an organization as having less moral standards due to the idea that it has lots of money All of this becomes difficult to handle for a single part-time employee funded at $8,000/yr by those $1 donations. From experience, all of this happens to charities. Even if you don't believe it, the fear of these risks is often greater than the salary earned by it. This is due to the unwritten law that "all transactions have a minimum and unavoidable customer support cost." At around $5, the above risks suddenly become worth the salary.