It would be nice to see an integration with modern bill for virtualmin. Modernbill (http://www.modernbill.com) currently offers a custom server module setting which i think Either joe or jamie can easily configure to work with virtualmin.
This is arguably the best billing solution out there. A solution which is affordable and most widely used. Agilebill although nice and all is not at all cost effective, and startup for agilebill is like 399+.
Please look into this. It would be deeply appreciated and i am sure will bring in a plethora of other users into using this software.