Our team did lots of stuff like that. Actually, I did big web-project for
rental booking and management, but you didn't hear that
To say what's better for you i need more details (which can be figured out if need contacting me on firstname.lastname@example.org
or IM you like).
anyway, here's a brief overview:
1. toolbars for browsers (we do it for IE and Firefox) - which has needed functionality (calendars/stored money transfer log and whatever), and which forwards you to your rental site if needed. for some cases there's no need to use database on server and for some there's (surely your hoster should support that)
2. stand alone application which does the same, but needs to be launched separatly
if you need more details, feel free to contact me.