As I'm convinced by your cart conceptI've spend some time with it to bring it to work within a ModX 0.9.6 installation with ditto 2.0.3 and PHx 2.1.3.
The addtocart snippet tells me: "Error: You must set your available and unavailable parameters" though I think to have installed everything as per instruction. The chunk "availabe" works so far that a cart apears with the appropriate data but when recalling the cart it is empty.
The page with the [!FoxyCart_Inventory!] call says "error: no post data".
It maybe a little I did not understand, because not post problems here.
As I'm no programmer I need a little xtra help on how to handle the test_xmldatafeed.php.
thanx for listening