Hi Kapitol
I'm answering myself because they didn't respond
If you look at the dozens of other e-Commerce cart or shop solutions, they pretty much all support Stripe, which in the last year or so seems to have rapidly become the payment provider of choice for small e-businesses starting out. Which surely is ShopSite's target demographic. Couple that with the fact that it looks like it would take an experienced developer an afternoon to integrate it, and it really makes me wonder why they haven't already.
Why do I want to switch to Stripe? Because I'm sick of merchant account providers constantly raising rates and adding spurious fees for this that and the other. Stripe has a standard published rate and no other fees.
In ShopSite's favor, I spent about three weeks looking at many other e-Commerce solutions and couldn't quite replicate the flexibility I have now. The breadth of customization ShopSite allows appears to be unique.