Shopping cart

I have six feature files for this exercise:
1. add_quantity.feature: Add Products to Shopping Cart
2. change_quantity.feature: Change Quantity of Products in Shopping Cart
3. remove_quantity.feature: Remove Products from Shopping Cart
4. coupon.feature: Add Coupons to Shopping Cart
5. navigate_to_item_pages.feature: Navigate from Shopping Cart to Item Page
6. shipping_estimate.feature:  Obtain shipping estimates
other test cases to consider: 

* what if someone tries to add a valid coupon multiple times?
* what if a coupon expires before the cart is checked out?
* how do coupons interact with shipping prices/estimates?
* how do coupons interact with one another? (for example, oftentimes stores will only allow you to apply one coupon at a time)