I work with WHMCS since 3 years ago (as an accounting manager or as a designer / programmer). I also have experience with ClientExec, FreshBooks, WHMAutoPilot, ModernBill, PHPAudit and other billing scripts. Also, I had been working as a support rep. (including billing) for 5 years (Bilingual: English / Spanish) on different web hosting sites (can provide experience and referrals), so I can take queries on the billing dept. with no problem. Have experience with Paypal also if needed.