A huge effort has been applied on the study of Wireless Mesh Networks (WMNs) over the last few years. As part of this effort the IEEE 802.16 Standard for Local and Metropolitan Area Networks specifies a mesh mode of operation. Although several technical aspects of the physical and medium access layers of the standard have been studied, only few works investigate how the WiMAX mesh mode architecture will handle billing aspects and paid access. Here we propose a billing architecture and present an accounting mechanism which allows WISPs to charge its users and also reward those who contribute forwarding packets. We evaluate the accounting mechanism through simulations and show its feasibility.