The total price that you see for a line item in the cart is calculated with the TotalPrice cart calculation. You can take a look at it in the cart object under the calculations tab. One really cool feature of eCart is that you can customize the calculations or make your own. When altering or crafting one you can make use of php code directly in the calculation.
I think that this will be the key for you to get the total to represent the additional line items in the cart. If you do it like this you will get the accurate price for the items in the cart itself and can rely on the built in cart functionality for discounts, charges, shipping, tax and all of the regular checkout stuff.
