I'll share it on apps market as soon as it's finished for free. That way, I can have my lessons and everyone else the final solution. Deal?

Hello. OK, @thomlin lets continue our lesson. :)

1. You need to register a handler for 'stripe_v3_create_session' alert from 'Payments' module. I'll use the following names and prefixes in my example:

• module's verndor - tester
• module name - bx_dummy
• module folder - dummy
• module class prefix - BxDummy

So, you need add the following code in enable.sql of your custom app.

INSERT INTO `sys_alerts_handlers` (`name`, `class`, `file`, `service_call`) VALUES
('bx_dummy', 'BxDummyAlertsResponse', 'modules/tester/dummy/classes/BxDummyAlertsResponse.php', '');
SET @iHandler := LAST_INSERT_ID();
INSERT INTO `sys_alerts` (`unit`, `action`, `handler_id`) VALUES
('bx_payment', 'stripe_v3_create_session', @iHandler);

and this one in disable.sql

SET @iHandler := (SELECT `id` FROM `sys_alerts_handlers` WHERE `name` = 'bx_dummy' LIMIT 1);
DELETE FROM `sys_alerts` WHERE `handler_id` = @iHandler;
DELETE FROM `sys_alerts_handlers` WHERE `id` = @iHandler;

2. Now we need to create BxDummyAlertsResponse class in modules/tester/dummy/classes/BxDummyAlertsResponse.php file and process the alert. So, the file should have the following code:

<?php defined('BX_DOL') or die('hack attempt');
/**
 * Copyright (c) UNA, Inc - https://una.io
 * MIT License - https://opensource.org/licenses/MIT
 * * @defgroup    The Art Collectors Club
 * @ingroup     UnaModules
 *
 * @{
 */

class BxDummyAlertsResponse extends BxDolAlertsResponse
{
    protected $_sModule;
    protected $_oModule;

    public function __construct()
    {
        parent::__construct();

        $this->_sModule = 'bx_dummy';
        $this->_oModule = BxDolModule::getInstance($this->_sModule);
    }

    public function response($oAlert)
    {
        $sMethod = '_process' . bx_gen_method_name($oAlert->sUnit . '_' . $oAlert->sAction);
        if(!method_exists($this, $sMethod))
            return;

        $this->$sMethod($oAlert);
    }

    protected function _processBxPaymentStripeV3CreateSession($oAlert)
    {
	$aSession = $oAlert->aExtras['session_params'];

	//TODO: Make your customizations here.

	$oAlert->aExtras['session_params'] = $aSession;
    }
}

/** @} */

So, in this case $aSession variable will contain the array which you wanted to modify and thefore you need to do something like the following:

$aSession = $oAlert->aExtras['session_params'];
$aSession['payment_method_types'] = array_merge($aSession['payment_method_types'], ['sofort', 'sepa_debit']);
$oAlert->aExtras['session_params'] = $aSession;

After that you need to disable/enable your app and test.