My institution is exploring the idea of implementing a new online giving form. Do you have any suggestions based on the requirements listed below. Ideally we'd like to find a service to work with that offers a turn-key system, but aren't opposed to the idea of working with a vendor to develop a custom solution.
Our institution uses Raiser's Edge CommunitySpark for this. I don't have direct involvement with that system, but at first glance but I don't think it offers the level of customization you are looking for.
I believe there are a lot of good solutions available in the non-profit/NGO space, and some of them may have the flexibility you are after. One starting point for researching these: http://www.idealware.org/articles/few-good-online-donations-tools.
Doug, I am looking for something very similar. Have you found a good vendor?