"it could be quite shocking or it could be quite gratifying"

An English publisher and author are experimenting with a "pay what you wish" model for the author's next book (a book which bears the perhaps unintentionally hilarious (in light of this initiative) title What Price Liberty?.  Which is an excellent idea, but one thing might make it even better: the publisher should add an option which allows purchasers to "top up" their payment afterwards.  In other words, it would be interesting to see if purchasers who decided to pay $0 or, say, $5 would, after downloading and reading the book, be willing to "top up" their payment - in effect saying "I thought I was going to get $x worth out of this purchase, but in fact I got $x+y - here's the $y".