“wpcargo_after_reciever_meta_section_sep” is a filter hook which will allow you to add a separator after the shipper and receiver details section.

Copy and paste the following code in your current theme’s function.php

function mycustom_after_reciever_meta_sep(){ ?>
    <style>
    .clear-line{border-bottom:1px solid #000;}
    </style>
    <?php
}

add_filter('wpcargo_after_reciever_meta_section_sep', 'mycustom_after_reciever_meta_sep' );

Screenshot: