SiteInvoice does require the Sitellite CMS framework to be installed, since it operates as an add-on to Sitellite. However, with a couple quick settings changes, you can essentially make SiteInvoice behave as a standalone application, ignoring the rest of Sitellite and letting you just use SiteInvoice by itself.

First, install Sitellite and SiteInvoice as per the usual steps.

Next, in Sitellite's control panel or in the file
inc/conf/config.ini.php change the default_handler setting to
'siteinvoice/index' and the default_handler_type to 'box'. This tells Sitellite to let SiteInvoice take over as the default request handler instead of the usual home page.

Now when you go to the site you've installed Sitellite and SiteInvoice on, without having to add the explicit /index/siteinvoice-app you'll still get SiteInvoice's login page right away.

One limitation of this is that SiteInvoice still relies on Sitellite for its user management (for any additional users who can log into SiteInvoice), but you can still access the rest of Sitellite by going to /sitellite as any ordinary site would for that.