the line should be
var selectedText = $(this).find("option:selected").text();
Change
var thisvalue = $(this + "option:selected").text();
to
var thisvalue = $("select#shipping_rates_drop option:selected").text();
Selected text value :
$('#selectBoxId').on('change',function(){ var optionsText = this.options[this.selectedIndex].text; alert(optionsText);});