The cart swf file can be loaded using the Loader class of AS3, the cart code will run flawlessly using this method.
Here's a sample code:
var myLoader:Loader= new Loader();
AS3-AS2 interactions are off course restricted to those allowed by the AVM1Movie Class, here’s an excerpt from Adobe flash CS3 documentation:
If the loaded content is an AVM1 SWF file (written using ActionScript 1.0 or 2.0), it cannot be cross-scripted by an AVM2 SWF file (written using ActionScript 3.0). However, you can communicate between the two SWF files by using the LocalConnection class.