The system should validate if the buyer can add product(s) to the Cart.

When the buyer tries to:

• add product(s) (variant) to the Cart
• increase the quantity of products added already to the Cart or
• proceed to the Checkout,

the system should validate if the requested product (variant):

• is available/in stock (*can we configure/customize if the out-of-stock product can be added to the Cart?)
• quantity is available
• base price in a specific currency is defined