Just discovered a bogus $4000 order by voice calling verification. Can never be too careful.I'll second HostLeet on the MaxMind and Voice Calling customers.
Here are a few things that I do (using WHMCS):
1. Use MaxMind fraud protection system.
2. Manually review every order before being approved.
3. Check and verify PayPal payments to make sure it matches the clients.
4. Put up a fraud warning for all fraudsters to see during the checkout process.
So far this has worked very well.