Um, no. The test needs to know the expected system behaviour, given that dependencies are properly wired & behave as expected. You can argue that "it no longer tests just the ShoppingCart in isolation" and that would be true; but whether that's a downside or an upside is up for debate IMO (and more often than not it really is an upside).
[edit] In other words, you don't test that "if discount is 10%, adding item to shopping cart increases cart total by 0.9×<price>"; you do test that "if user X adds item Y to shopping cart, and X has a 10% discount code, then shopping cart price increase by 0.9×<price>". This does seem like a headache, but it has benefits - in that it tests actual product scenarios, and you can now easily add variations like "if discount code only applies for books, and item I added isn't a book, then shopping cart total increases by <price>".
[edit] In other words, you don't test that "if discount is 10%, adding item to shopping cart increases cart total by 0.9×<price>"; you do test that "if user X adds item Y to shopping cart, and X has a 10% discount code, then shopping cart price increase by 0.9×<price>". This does seem like a headache, but it has benefits - in that it tests actual product scenarios, and you can now easily add variations like "if discount code only applies for books, and item I added isn't a book, then shopping cart total increases by <price>".