javascript add/remove text in div when checkbox is unchecked/checked

I am trying to learn the magic of Javascript. What I am trying to do here is to add / remove text in div when checkbox is unchecked / checked.

This is me trying to do somekind of shopping cart:

function addCartItem(checkboxElement,cartElement,cartItemElement){

    var s = "";
    if(checkboxElement.checked == true){
        s = document.getElementById(cartItemElement).innerHTML += checkboxValue;
        updatePrice();
    }
    else{
        s = document.getElementById(cartItemElement).innerHTML = checkboxValue;
        s = s.replace(s,"");
        updatePrice();
   }
}

I cant get this to work, it wont remove the value from the unchecked checkbox.

UPDATE I have this checkbox calling function like this:

<input type="checkbox" onclick="addCartItem(this,'cartElement','cartItemElement');" value="StarBreeze: TWD Demo |1337">