Started Alerts and the cron php script to trigger the alerts as well as check payment amount is greater than balance in invoice

add_payment_modal.php

@@ -9,6 +9,7 @@
       </div>
       <form action="post.php" method="post" autocomplete="off">
         <input type="hidden" name="invoice_id" value="<?php echo $invoice_id; ?>">
+        <input type="hidden" name="balance" value="<?php echo $balance; ?>">
         <div class="modal-body">
           <div class="form-row">
             <div class="form-group col">