The cart display template uses cartitem.product.unit_price (lines 51, 52)
This is a problem for any cartitems with details that adjust the price. It seems like it would be better to use cartitem.unit_price.
However, there is some existing logic for sales and discounts that might complicate things. Maybe we'd need to use cartitem.unit_price and cartitem.undiscounted_unit_price to get the same behavior.
I attached a patch that I think does what we need. It could probably use review before applying though.