in the add to cart button behavior, set the description to be updatable from a hidden field.
then add an onchange event to the price select list to populate the description hidden field using the syntax:
onChange="document.<formName>.<hiddenElementName>.value = document.<formName>.<listName>[document.<formName>.<listName>.selectedIndex].text"
where:
<formName> is the name of the form (or in your case eCart1_1_ATC_0
<hiddenElementName> is the name of the description hidden element
<listName> is the name of the price select list ( or in this case eCart1_1_Price_Add)