the cart object you have sent does no exhibit the error you reported in the initial post, in the cart object you have sent, no matter what I set the quantity to for an item, the item total is always the price.
If i have an item that costs $4 and add it to the cart, then change the quantity to 10, the total is still $4. This is because you have changed the TotalPrice calculation to:
it should be:
TotalPrice: [Price] * [Quantity]