Yes you have to have a license, it's $15.95 from www.whmcs.com
BUT you can get it for $12.95 (I believe) at www.licensepal.com
.. Which they also have cheapier clientexec licenses. Yes you can pay monthly at both of those websites. But I would suggest paying the extra $3 a month because if you order from a reseller they will not give you support. Also if you order from WHMCS directly their giving out free enom accounts.