The Shopping Cart lets users add, remove, and check items out.
They can add an item that wasn't already added.
The checkout process returns a receipt that holds the subtotal, tax, and total.

               Test Cases
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- adding an item if not already added
    - Test if the cart contains the item after it's added.
    - Test if the cart skips duplicate item.
- removing an item
    - Test if the item gets removed after being sold.
    - Test for IllegalStateException for trying to remove from an empty cart.
- checkout
    - Test if subtotal is calculated correctly.
    - Test if tax is calculated correctly.
    - Test if total is calculated correctly.
    - Test for an IllegalStateException for trying to checkout an empty cart.