PHP mail function how to set sender mail id based on if condition?

I am using magento for sending mail with condition,

My code:

<?php
class Gta_MerchantNotification_Model_Observer {
    public function merchantremainder($Observer) {
       $order = $Observer->getEvent()->getOrder();
       $order_details = $order->getAllVisibleItems();

       $itemData = array();
       foreach ($order_details as $list) {
          $incrementid = $order->getIncrementId();
          $sku = $list->getsku();
          $name = $list->getName();
          $price = $list->getPrice();
          $Qty = $list->getQtyOrdered();
          $extra = $order->getIncrementId();
          $message = 

          "
          <tr>
          <!-- <td>$incrementid</td> -->
          <td>$sku</td>
          <td>$name</td>
          <td>$price</td>
          <td>$Qty</td>
          </tr>";

          $itemData[$list->getId()] = $message;

      }

      $finalMessage =  "
      <p>Order Id : $incrementid</p>
      <table border='1'>

      <tr>
      <!-- <th>Id</th> -->
      <th>Sku</th>
      <th>Product name</th>
      <th>Price</th>
      <th>Qty Ordered</th>
      </tr>";
      if (!empty($itemData)) {
          foreach ($itemData as $data) {
             $finalMessage .= $data;
         }

         $finalMessage .= "</table>";
         $this->sendMail($finalMessage);
     }

 }

 public function sendMail($message) {
   $body ="$message";
   $emailTemplate = Mage::getModel('core/email');
   $emailTemplate->setFromName('abc');
   $emailTemplate->setBody($body);
   $emailTemplate->setSubject("Custom Email from observer");
   $emailTemplate->setType('html');
   $emailTemplate->setToEmail('abc@gmail.com');
   $emailTemplate->send();

}
}

?>

Output :

https://snipboard.io/encLQH.jpg

If order placed mail send to abc@gmail.com.
How to set email sender based on SKU $sku from order.

I want :

1) If SKU starts with 2, email should go to the mail id abc@gmail.com,
screenshot :

https://snipboard.io/2aiBze.jpg

2) If SKU starts with 3, email should go to the mail id xyz@gmail.com,
screenshot :

https://snipboard.io/tmWfEx.jpg

3) If SKU starts with 4, email should go to the mail id qwe@gmail.com,
screenshot :

https://snipboard.io/M5dGIn.jpg

FYI - If an order contains 10 items email should go separately based on SKU. But an order id the same must include all the emails.

How to get my value if drop-down filed using AJAX?

I have predefined value like shipping_weight = 10$, my form consisting two dropdown list, if the dropdown selected based on the selection how to return my ajax value,

Index.php :

    <tr>
        <th>I have : </th>
        <td>
            <select id="old" name="i_have">
                <option value = "select_option">Select Option</option>
                <option value = "three_compact">3 Compact</option>
                <option value = "three_regular">3 Regular</option>
                <option value = "three_triple">3 Triple</option>
                <option value = "five_compact">5 Compact</option>
                <option value = "five_regular">5 Regular</option>
                <option value = "five_triple">5 Triple</option>
                <option value = "seven_compact">7 Compact</option>
                <option value = "seven_regular">7 Regular</option>
                <option value = "seven_triple">7 Triple</option>
                <option value = "nine_compact">9 Compact</option>
                <option value = "nine_regular">9 Regular</option>
                <option value = "nine_triple">9 Triple</option>
            </select>
        </td>
    </tr>

    <tr>
        <th>I want : </th>
        <td>
            <select id="new" name="i_want">
                <option value = "select_option">Select Option</option>
                <option value = "three_compact">3 Compact</option>
                <option value = "three_regular">3 Regular</option>
                <option value = "three_triple">3 Triple</option>
                <option value = "five_compact">5 Compact</option>
                <option value = "five_regular">5 Regular</option>
                <option value = "five_triple">5 Triple</option>
                <option value = "seven_compact">7 Compact</option>
                <option value = "seven_regular">7 Regular</option>
                <option value = "seven_triple">7 Triple</option>
                <option value = "nine_compact">9 Compact</option>
                <option value = "nine_regular">9 Regular</option>
                <option value = "nine_triple">9 Triple</option>
            </select>
        </td>
    </tr>

    <tr>
        <th>Shipping Weight : </th> 
        <td id="results"></td>
        <input type="hidden" name="shipping_weight" id="shipping_weight"/>
    </tr> 
    <tr>    

    <script>
        $(document).ready(function(){
        $('#new').on('change',function(){

        var ship_val = $("#est_shi_val").val();
        $.ajax({
        type: "POST",
        url: "ajax_ship_data.php",
        dataType: "text",
        data: {est_shi_val: ship_val },
        success: function(data)
        {
            // Check the output of ajax call on firebug console
            // console.log(data);
            $('#results').html(data);
            $('#shipping_weight').val(data);
        }
});

});
});

</script>

My value is,

ajax_ship_data.php

if(isset($_POST['old']) && isset($_POST['new'])){
$old = $_POST['old'];
$new = $_POST['new'];

// three step kolupadi combination 

//part 1 

if($old=='three_compact'&&$new=='five_compact'){  // 6250111

/** echo json_encode(array('Cost 10$')); */
echo "10";
}

How get my shipping weight if two drop down seleted.
FYI -> https://snag.gy/pNaEMv.jpg