Peer-to-peer (P2P) anonymous communication systems are vulnerable to free-riders, peers that use the system while providing little or no service to others and whose presence limits the strength of anonymity as well as the ef- ficiency of the system. Free-riding can be addressed by building explicit incentive mechanisms into system protocols to promote two distinct aspects of cooperation among peers - compliance with the protocol specification and the availability of peers to serve others. In this paper we study the use of payments to implement an incentive mechanism that attaches a real monetary cost to low availability. Through a game theoretic analysis, we evaluate the effectiveness of such an incentive, finding that peer availability can be significantly increased through the introduction of payments under many conditions. We also demonstrate how a payment-based incentive that preserves anonymity can be implemented and integrated with a popular class of P2P anonymity systems.