We recently added a new option for event organizers to enable donors and ticket buyers to choose to pay the ticketing fee for donation ticket types. Below is a screenshot of the add-to-cart modal for donation ticket types with this feature enabled.
To enable donors and ticket buyers to choose to pay the ticket fee for donation ticket types, visit the create/edit tickets page for your event from the manage (shared) events page. Create a new ticket type or edit an existing one. Select donation as the ticket type, and “Ticket buyer decides” from the ticketing fees drop-down, as seen in the screenshot below.