[Suggestion] - Allow different product model

[thomascenni]

Hello,
With the actual implementation of one CART_PRODUCT_MODEL, it is impossible to add to the shopping cart an item like a "shipment cost", because this is something you calculate based on some inputs and is not a product (should not be stored in the same product Model).
As a workaround, I am using two carts, one for the products and one only for the shipment, where the shipment uses a different product model. I had to modify Cart class and also the template tags.
The suggestion would be to use the variable "session_key" not only to address different carts of the same model (as today), but to point directly to the model you want to use. Do you think it is possible ? Do you have any other suggestion ? Thanks