I have been doing some testing of this and it seems that there is a problem with the force decimals option. In the files that you sent on line 278 of the cart page you have this for the definition:
$WA_Store_Cart = new WA_Store_Cart_Definition("WA_Store_Cart", 0, "€", "", false, 4, 0, "kg", false, 4, 1, 0);
In this code it is the false after the "", that is the setting for force decimals. When you check your eCart object what do you see in the interface? Make sure that the box for force decimals is checked. It should update this code to include decimals out to 4 places. Please give this a try and let us know what you experience.