Manual for ISPCbilling-system ? Does it giva a manual in german for the ISP-Faktura (it´s german - I mean the billing-System of ISPC) ? If not, I will try to write one (by try an error and with help from the dev-team )
Have a look here: http://www.projektfarm.com/fileadmin/downloads/manual_full_de.pdf (chapter 7). It's for 42goISP, but it applies to ISPConfig as well.