|Perl: the Markov chain saw|
But before you use Interchange, you might read my explanation of why I dumped Interchange.
Note it's been a couple years and I'm not sure what's changed with Interchange.
My current bias that I tend to write smaller e-com applications and the re-usable applications I find tend to be "too much" and are harder than needed to work with and maintain because of the additional complexity.
Although e-commerce is complex, it's a collection of things are all fairly straightforward these days: using a database, session handling and posting over SSL. Definitely because ther are financial transactions involved, I think there is more of emphasis (and programmer fear!) of quality control and "getting it right".
In my own cases I've found that custom solutions have been a good fit to build and maintain.
That said, I would happily consider adopting an e-commerce package if it used some my favorite tools:
The right fit for me would allow me to use just the bits I need when I want them, and also have more power there when I need it....much like a collection of fairly independent Perl modules. :)