I don't have immeidate availability to the code base, but I do know
how this was accomplished by our contractor/vendor who integrated
their shopping card with uStore. Basically the load the uStore
session into an iframe then with a hook into the the frame's load,
checking for either the submit or cancel URL. Prior to serving the
iframe up, they are tracking the locally associated product with a
session variable, so when the app hit the submit URL, it will add the
local product to the local cart (versus the uStore cart).
For example, you have "My Business Card" with a product code MY-BC on
your site which stores a uStore Product ID, 1234. When the user
chooses to start customizing you push MY-BC into a Session variable,
lauch the iframe with uStore, and hook in the iframe's load. Check
time a page loads it will evaluate if the product was added to cart
(also look for the cancel, to close the iframe). If so, it will close
the iframe, and add MY-BC to the local cart. This matches the uStore
card with product 1234 in it.
When submitting they also need to programatically comit the uStore
cart as well (it was decided to only do so when there was something
acutally in the uStore cart, which is again tracked within the local
app). They choose to create a uStore order when a person first signs
on. They continue to use that order until it is placed, the create a
new order immeidate afer placing. This has the effect to always have
an assocaited uStore cart available.
Hope that helps.
p.s. In looking that the API for another project, I think it may be
possible to