Can I override shoppingCart?

I want to modify the class so the method "reset" doesn't remove products of a certain type ("rental_product"). Only the customer can remove these products from the cart.
Then I can have a script which every month generate an invoice based on the products of that type in the cart.

What other modification to the the shoppingCart am I supposed to do in order to make that work?

The reason why I want to use shoppingCart also for inventory is that it handels attributes and pricecalculation so nicely.