I had the same issue and found this answer just now.
According to PayPal (
cms.paypal.com/cms_content/US/en_US/files/merchant/paypal_nonprofit_faqs.pdf), donation fees are 2.2% + $0.30. So a $2 donation nets lichess ~$1.65. I think that's "significantly more" than $0.
I agree with leonarth; if you're going to set a minimum for giving the trophy, you should list it or limit the donations somehow. But it's not like the trophy costs you anything to give out anyway...