Simple sales plugin

Can someone help me out here?

I've got a new plug-in I'm going to try sell on the warrior forum. In the past, I've used Mike Filsaime's butterfly script to sell single digital products and it has worked well, but I'm looking for a wordpress solution that:

1. Is designed for wordpress and so integrates properly (some things like DAP and SMP are not designed for wordpress and so are awkward to use)
2. Integrates with multiple payment gateways, preferably including the popular WSO platform or at least has an open API to add the WSO one
3. It would be nice if it integrated with Amazon S3, but not a requirement
4. Ability to create OTOs would be nice, but not a requirement
5. Integration with autoresponders would be nice, but not a requirement

In other words, it should be something like the shopping cart or membership plug-in, but much simpler with just one product and perhaps support for one or two upsells or downsells.