EventBright is the only ticketing service which offers a ticket by donation service that I know of, if anyone knows of another one please let me know.
Using EventBright is something of a learning curve, as they don't tell you, clearly up front, about how the ticketing charges levied. I don't particularly like the way in which it works, but now that it has been set up I am stuck with it.
The issue of extra charges being levelled is because the suggested donation has been set below £5. Above £5 and the whole charge comes off the donated amount and there appear to be are no up front charges to the ticket buyer. The event organiser gets charged about 15% of the total donated amount by EventBright.
The reason the event is ticketed is so that I can find out before hand how many people are likely to come. It is also much quicker getting people into the venue if people just show a ticket. There is a very tight time scale on this talk.
Yes you can come and give a donation on the night (or not as you please), but those holding tickets will have priority for entry.