i2p package will be installed in /opt/i2p and an additional rc file called
rc.i2p will be putted on /etc/rc.d directory.
If you want to change installation dir, change the variable INSTALL_DIR
on i2p.SlackBuild and rebuild the package.

Probably you will never have to update i2p packages with upgradepkg,
just because i2p have an auto-update function.
How to start i2p:
After installpkg command, doinst.sh will execute a postinstallation script
needed by i2p. After that you have to chmod +x /etc/rc.d/rc.i2p and start
i2p service with /etc/rc.d/rc.i2p start.
Now tell your browser to user this proxy: localhost on port 4444 and open
this page: http://localhost:7657/index.jsp
Here you can configure i2p, watch network status and navigate anonimously.
It's suggested to subscribe to various dns host, like i2phost.i2p
For any additional information, visit i2host.i2p and forum.i2p

If you want to start i2p at boot you have to chmod +x /etc/rc.d/rc.i2p and put
the following line to /etc/rc.d/rc.local:

 /etc/rc.d/rc.i2p start

To stop i2p at shutdown you have to create a /etc/rc.d/rc.local_shutdown and
put inside this file the following line:

 /etc/rc.d/rc.i2p stop