As part of the RSVP confirmation process, attendees can be offered the option to pay online, with secure payment processing by PayPal. Even if they don't have a PayPal account, they can enter a credit card number and PayPal will handle the secure payment processing.
Here is an example from the event registration for a Society of Professional Journalists regional conference:
RSVPMaker.com is a service of Carr Communications Inc.
David F. Carr