AddProductToCart adds a product to the cart of a user.

BuyItemFromCart fetches the cart of the user, finds the cart total, creates an order with the items, adds the order to the user collection, adds the items in the cart to the order list, and empties the cart.

DBSet creates a new MongoDB client and connects to the database.

InstantBuyer adds a product to the cart of a user.

RemoveCartItem removes an item from the cart of a user.