Magento – Onepage Checkout – Get selected payment method

You can do this by Magento event observer functionality whenever magento predispatch a controller then trigger event controller_action_predispatch_youfullaction

Mage::dispatchEvent('controller_action_predispatch_' .
    this->getRequest()->getRouteName(), array('controller_action' => $this));

you need trigger an event when shipping method is select and click save button to goto next button you need event and redirect to third party from observer.

Here is an example that depends on Magento default

checkout onepage and trigger event on
controller_action_predispatch_checkout_onepage_saveShippingMethod

<frontend>
        <events>
            <controller_action_predispatch_checkout_onepage_saveShippingMethod>
                <observers>
                    <my_fire_events>
                        <class>magento38137/observer</class>
                        <method>myredirection</method>
                    </my_fire_events>
                </observers>
            </controller_action_predispatch_checkout_onepage_saveShippingMethod>
        </events>
</frontend>

I got an answer to this same question on StackOverflow by user Elvarasan. His answer provided me with everything I needed to get this working.

To hide some payment methods based on shipping method you don't need to observe things at all:

Every methods(in one page check out) post the methods which are chosen to the next level. So you can get the shipping method which are chosen, in payment method level. Just print the post things in

app/design/frontend/base/default/template/checkout/onepage/payment/methods.phtml

in this add below one,

<?php print_r($_POST); ?>

So now you can get the shipping methods which are chosen previous step. And note it, so now, you can add just simple logic (if else) condition in same file for hiding payment.

For example here I want hide check / money order payment method, if shipping method is flat. Here the payment method code is checkmo. you can get payment method code by simply printing that variable like echo $_code = $_method->getCode(); in same file. so here just add simple if else ,

<?php
    $methods = $this->getMethods();
    $oneMethod = count($methods) <= 1;
?>
<?php if (empty($methods)): ?>
    <dt>
        <?php echo $this->__('No Payment Methods') ?>
    </dt>
<?php else:
    foreach ($methods as $_method):
       echo  $_code = $_method->getCode();


if($_POST['shipping_method'] == 'flatrate_flatrate') {
    if($_code == 'checkmo') {
        continue;
    }
}
?>

Here,

if($_POST['shipping_method'] == 'flatrate_flatrate') {
    if($_code == 'checkmo') {
        continue;
    }
}

checks the shipping method and skip the payment method which we don't want to display. That's it. Please comment here if you have any doubt.

Note:

 shipping_method => flatrate_flatrate
 payment_method   => checkmo