Subtotal Is not Implemented on Cart Page

[sbmsr]

Description

When a user adds something to their cart, the cart page subtotal is static. We need it to dynamically update when a user adds something to their cart.

How To Reproduce

1. Run the App Locally
2. Add item to cart
3. See that cart subtotal is $262.

Deliverables

• Fix cart subtotal to accurately reflect items in cart