Custom Shopping Cart (ecommerce integration), for service based business

We are looking to add shopping cart software to our website. Currently, we have Zen Cart installed (/shop), however, we would entertain any of the other open source software if they fit our requirements more specifically....so feel free to make a suggestion. We offer services not products, so this would not be an extensive shopping cart...should be pretty basic. Here's a general overview of what we do, and what we would like the shopping cart to do: - We offer online conferencing services - we would like our users to be able to book a meeting online through our website - when booking this meeting, the user would: select the service, choose the date and time, duration of the meeting, and select any other add-on options for this meeting - each service ( 5 services in total), will offer different methods of sign-up: (1) Book an individual meeting (2) Subscribe to monthly payment (3) Or sign-up for pay-per-use account Here's what we would like to do with the shopping cart: - Custom CSS look and feel to match our website - Integrate our payment processing company (moneris) - have ability to sign-in, so their last purchase is remembered (however, if possible we do not want this to be mandatory to make purchase) - Create custom options in shopping cart, such as, creating a date and time select field - Pre-project consulting to determine best layout ( 2 hour online meeting) For now we are satisfied with an email notification, when a new order arrives. We also expect to manage the shopping cart at end of project, such as, add additional options, or additional services, so a 1 hour consulting session online would be expected.