Ask HN: Advice in collecting dues from a membership organization/club?

I help out with a 500 member organization, an alumni-type of group. Everyone in this organization is a volunteer. We don't have a lot of funds at the moment, we are just now moving from a loose unorganized collection of people with a similar background to an actual orgnization.

* Can you please recommend a service to automate the collection of dues from members of an organization? We'd like to offer them the option of having a $5 - $20 per month taken out of their checking account. Obviously participation is voluntary and opt in.

I have seen what PayPal and BlackBaud have to offer. Other options? Can we get a better rate than having the provider take 4.9% and $.30 per transaction?

I'd love to hear your recommendations and experience if you have had it in this area.

Thanks!