Tags being Inserted in html from plugin PHP not passing through the_content

While I don’t think anyone can solve the source of the issue without poking around the codebase, I think the (untested) code below may solve the impact.

Use output buffering to store the <script>, and then remove all the new lines and extra whitespace (tabs, line breaks, multiple spaces).

function i13woo_extra_checkout_fields() {
    ob_start();
    ?>

    <script>
        ...
    </script>

    <?php
    $script = ob_get_return();
    $script = str_replace( array( PHP_EOL, “\t”, '  ' ), '', $script );

    echo $script;
}