If we have a 'pending' payment for Mollie within a transaction, use that payment to run the process_ipn function and confirm with Millie if the payment is complete. If that payment is complete it will update and trigger any messages normally sent.

example.php

<?php //Please do not include the opening PHP tag if you already have one

add_action(
    'AHEE__Transactions_Admin_Page__transaction_details__start',
    'tw_ee_check_for_mollie_payment_update_on_transaction',
    10,
    1
);

function tw_ee_check_for_mollie_payment_update_on_transaction($transaction)
{
    if ($transaction instanceof EE_Transaction) {
        // are there pending Mijireh payments on this transaction?
        $a_mollie_payment = EEM_Payment::instance()->get_one(
            array(
                array(
                    'TXN_ID' => $transaction->ID(),
                    'STS_ID' => EEM_Payment::status_id_pending,
                    'Payment_Method.PMD_type' => 'Ideal_Mollie',
                ),
            )
        );
        if ($a_mollie_payment instanceof EE_Payment) {
            add_action(
                'AHEE__EE_Registration_Processor__trigger_registration_update_notifications',
                'tw_ee_send_notifications_after_mollie_ipn',
                5,
                2
            );
            EE_Payment_Processor::instance()->process_ipn(
                array('id' => $a_mollie_payment->txn_id_chq_nmbr()),
                $transaction,
                $a_mollie_payment->payment_method()
            );
        }
    }
}


function tw_ee_send_notifications_after_mollie_ipn($registration, $additional_details)
{
    $last_payment = isset($additional_details['last_payment']) ? $additional_details['last_payment'] : null;
    if (! $last_payment instanceof EE_Payment || $last_payment->status() != EEM_Payment::status_id_approved) {
        add_filter('FHEE__EED_Messages___maybe_registration__deliver_notifications', '__return_false', 15);
    }
}