I guess that this counts as extreme cutomising. I have a site where most pages are written in something other than php - but which can make http calls (get or post) to the zen cart store I am setting up.

I want to embed a couple of things on these pages:
1) a product selector (dropdown with all the size variations for this product) and add to cart form.
2) A summary of cart contents.

So, I need to write php scripts which initialise the system, do all the back-end work, then spit out only the HTML for the desired components.

Is there an existing system for doing this?