I have recently begun doing a bunch of sites for small/medium businesses and I host them on my own VPS account. Up until now I have been invoicing my customers manually but I would like a more automated solution because it's starting to become a pain. Also, with some of the customers, such as restaurants, I trade the hosting for gift certificates at the restaurant so I need to be able to keep track of those as well.
What do the rest of you use for doing this?