How to fundraise for nature lovers small group

You're the leader of a common interests small group that's just for nature lovers at your church. You've got a great six week series lined up, but it revolves around a new book that you know not many of the others have. Your group's small budget doesn't cover the costs, so you decide to raise the funds by taking donations from within the group. You'd like to do it online, if possible, because you don't want anyone to feel pressured at the meetings.