输入错误代码时出现php折扣代码错误

Question

我正在尝试将折扣代码集成到我的购物车中。它工作正常，在输入代码时计算正确的折扣，当用户输入错误的代码时我遇到的问题，购物车总数刚刚变为0.00英镑。有人能指出我正确的方向吗？

javascript：

$(document).ready(function(){
$("#addcoupon").click(function(){
    var ttlamt = $("#ttl_amount").val();
    var ccode = $("#couponcode").val();
    var cpn = ccode.replace(/\s/g, '');
    if(cpn.length <= 0){
        $("#couponcode").css("border-color","red");
    }else{
      $("#couponcode").css("border-color","");
      $.ajax({
          dataType: 'json',
          url: "page.php",
          method: "POST",
          data: {action:"coupon_dis",couponval: cpn,amnt: ttlamt},
          cache: false,
          success: function(data){
            if(data != "0"){
                $("#grandamount").css("display","none");
                $("#discount_amount").css("display","none");
                $("#couponcode").css("border-color","green");
                $("#grand_discount_amount").css("display","block");
                $("#final_discount_amount").css("display","block");
                $("#grand_discount_amount").html(data.price);
                $("#final_discount_amount").html(data.discount);
            }else{
                $("#discount_amount").css("text-decoration","");
                $("#grandamount").css("text-decoration","");
                $("#couponcode").css("border-color","red");
                $("#grand_discount_amount").css("display","none");
                $("#final_discount_amount").css("display","none");
                $("#grand_discount_amount").html("");
                $("#final_discount_amount").html("");
            }
          }
      });
    }
    return false;
});

page.php文件

if(isset($_POST['action']) && $_POST['action'] == "coupon_dis")
{
   $couponcode = trim($_POST['couponval']);
   $amnt = $_POST['amnt'];
   $val = '0';
   if($couponcode == "loyalty10"){
      $couponDiscount_val = "10";
      $discount_amount = floor($amnt*$couponDiscount_val)/100;
      $amount_after_discount = $amnt - $discount_amount;
      $val = sprintf("%01.2f", $amount_after_discount);
}
   $_SESSION['discount_amount'] = sprintf("%01.2f", $discount_amount);
   $_SESSION['product_discount'] = $val;
}

$return = [
   'price' => $currency.$val, 
   'discount' => $minus.$currency.number_format((float)$discount_amount, 2,'.', '')
];

header('Content-Type: application/json');
echo json_encode($return);