aeserver, Plesk is a control panel, not a billing solution. SW-Soft's billing solution, HSPComplete, starts at around $5000 and is geared for people using Plesk. However, it's a very very very nice system.
AWBS (www.awbs.com) is also a very stable and usable client which supports cpanel, directadmin, paypal, authorize.net, and all that stuff.
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