Paypal charges 2.9%+$0.30 per transaction. Currently, if you need to refund a buyer or donor for any reason then Paypal keeps the $0.30, but they refund the 2.9% fee. Under the new terms they will no longer refund the 2.9% fee.
That means a returned item will hurt sellers even more than it did in the past.
The only way to opt out of this fee is to close your Paypal account before 5/7.