I would say depends how in demand you are. How well known is your writing? Is your book on sale too?
I say maybe leave it as an open contribution.
Now that I think about it one thing you could do is define a goal that you will aim to reach, and update these donators on how you are getting along. Like "I am half way to my $2,500 mark that I will use to do whatever".
This may encourage people to donate more than one time.