IMHO there is no good solution to solve those issues with volume based services. You can just be generous to your customers like we do. Maybe we will hear soon how sunrise handles such cases on their ADSL services with dynamic IPs ;-)
There are solutions, like force the user to use a socks 4/5 proxy you would provide and do the accounting on it. however, the user needs some knowledge... to configure its softwares.. and not all applications support that (or ignore the global Windows settings)..
Pascal