your product recordset is filtered on a posted form element named ID which does not exist when the add to cart button is pressed.
normally, a product detail page would use a querystring variable instead of a posted form variable.
change:
<input type="hidden" name="eCart1_1_ID_Add" value="101" >
to:
<input type="hidden" name="eCart1_1_ID_Add" value="101" >
<input type="hidden" name="ID" value="101" >