We are building an online subscription-based website and I'm looking for recommendations on which e-commerce platform to use for the checkout process.

Requirements include:

1. Only four products.
2. The sign-up process of the site is heavily customized, and after checkout the user should automatically get logged into the subscriber area.
3. Subscriptions will last one year and can be renewed manually.
4. Support for coupons/discount codes at a later point.

Since the entire application is custom, we've weighed building a custom checkout but are strongly leaning towards using existing software to avoid having to build lots of admin reporting as well as a coupon engine down the road.

The two questions we're pondering are:

• Is it better to a) build our application custom and use whatever e-commerce software we select just for the payment piece, or b) use the e-commerce software as the basis and build our application around it/as a module/etc?

• Which e-commerce platform should we use? I've looked into a variety of off the shelf e-commerce software, but it's not clear to me which would be easiest to integrate with. I've researched on the Web and looked at many of the threads on SO to compiled a list of potential candidates:

www.magentocommerce.com/ (seems difficult to integrate with)
www.prestashop.com/
www.nopcommerce.com/
www.opencart.com/
www.cubecart.com/
www.spreecommerce.com/
www.interspire.com/
www.tradingeye.com/

We're most concerned with the level of effort required with ramping up on the software and then doing the integration with our custom functionality. We're most proficient with PHP, ASP.NET and some ROR and are only considering those technologies. We prefer open source, but would be open to commercial if there's a significant upside.

Any experiences with similar projects and advice is greatly appreciated.