How to redirect the user to a product page (node) after payment is made?
One option is to use hook_menu_alter in a custom module to override your current destination page with a redirect. 

&lt;?php

function mymodule_menu_alter(&amp;$items) { 

    $items['current/checkout/path'] = array(
        'page callback' =&gt; 'drupal_goto',
        'page arguments' =&gt; array('new/checkout/path'),
        'access callback' =&gt; TRUE,
      );

}


Update:

To use the above code, you'll have to create a module.

1) Create a directory to be used for this module, e.g. /sites/all/modules/MYMODULE

2) Create MYMODULE.info, e.g. /sites/all/modules/MYMODULE/MYMODULE.info. Paste the following into the file.

name = MYMODULE
description = "Module description."
package = Custom
core = 7.x


3) Create MYMODULE.module, e.g. /sites/all/modules/MYMODULE/MYMODULE.module. Paste the following into the file.

&lt;?php

function MYMODULE_menu_alter(&amp;$items) { 

    $items['current/checkout/path'] = array(
        'page callback' =&gt; 'drupal_goto',
        'page arguments' =&gt; array('node/123'),
        'access callback' =&gt; TRUE,
      );

}


4) Replace all instances of MYMODULE with your desired module name. Replace current/checkout/path with the current path of the checkout. Replace node/123 with the new checkout path. Lastly, enable your new module.

This will tell Drupal to replace the current checkout page with a redirection to the new one.