A hook for WHMCS which disables email notifications for invoices with a zero total. Modified from existing code to use the new Capsule database handler ( http://www.whmcsjet.com/how-to-disable-invoice-generation-for-0-00-invoices/ ).

disable_zero_invoices.php

<?php
if (!defined("WHMCS"))
    die("This file cannot be accessed directly");

use WHMCS\Database\Capsule;

function disable_00_invoices($vars) {
    
    $merge_fields = array();
    $email_template_name = $vars['messagename']; # Email template name being sent
    $relid = $vars['relid']; # Related ID it's being sent for - client ID, invoice ID, etc...

    //Checking for certain template name, if so - this is our case
    if ($email_template_name == "Invoice Created" || $email_template_name == "Invoice Payment Confirmation"){
        //getting total of the invoice
        $inv_total = Capsule::table('tblinvoices')->where('id', $relid)->pluck('total');
        if( is_array($inv_total) ){
           $inv_total = $inv_total[0];
        }
        //if it is equal to '0.00' we disable email sending
        if ( null !== $inv_total && '0.00' == $inv_total )
            $merge_fields['abortsend'] = true;
    }

    return $merge_fields;
}
 
add_hook("EmailPreSend",1,"disable_00_invoices");