Open source online registration system that collects information and payment

I am currently working with a client who offers a vacation (or more of a mission trip) on their website. Our goal is to have a method for collecting a bunch of information - desired travel date, passport info, medical, etc. as well as a $275 payment. We currently have an Authorize.net account. We'd like to keep all of the functions on our website so that it looks seamless. Additionally, we haven't much money to spend on the development process, so we're looking at open source options. I tried OSCommerce and ZenCart, but they are too much of a shopping cart, and not so much of an information system. I'm sure if I knew more about them they would work to "customize" and use, too, but I also have very little time. I did extensive searching for "event management" or "event registration" products, as I thought those would suit our needs better, but the only thing that I found is something called EVS Minder (http://www.evsminder.com/) and I can't find a download or anything even though it appears to be open-source.

Do you know of anything that suits the primary need of data collection and storage as well as the secondary need for payment processing through Authoirze.net.

Thank you!