This is dead code, which was never added to the core of Tiki. Instead use the Payment feature, which handles invoices.
Mod tinvoiceTinvoice mod intend to add invoice management and generation to Tikiwiki.
It's under heavy development currently (summer 2007), and it requires Tiki 1.10, php5 and ajax.
InstallSee Mods Admin for instructions on how to install a Mod.
UsageNot ready for production yet.