You can use this custom function hooked in woocommerce_cart_calculate_fees
action hook, that will make a discount of 15% for a defined payment method.
You should need to set in this function your real payment method ID (like 'bacs', 'cod', 'cheque' or 'paypal').
The 2nd function will refresh checkout data each time that a payment method is selected.
The code:
add_action( 'woocommerce_cart_calculate_fees','shipping_method_discount', 20, 1 );
function shipping_method_discount( $cart_object ) {
if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;
// HERE Define your targeted shipping method ID
$payment_method = 'bacs';
// The percent to apply
$percent = 15; // 15%
$cart_total = $cart_object->subtotal_ex_tax;
$chosen_payment_method = WC()->session->get('chosen_payment_method');
if( $payment_method == $chosen_payment_method ){
$label_text = __( "Shipping discount 15%" );
// Calculation
$discount = number_format(($cart_total / 100) * $percent, 2);
// Add the discount
$cart_object->add_fee( $label_text, -$discount, false );
}
}
add_action( 'woocommerce_review_order_before_payment', 'refresh_payment_methods' );
function refresh_payment_methods(){
// jQuery code
?>
<script type="text/javascript">
(function($){
$( 'form.checkout' ).on( 'change', 'input[name^="payment_method"]', function() {
$('body').trigger('update_checkout');
});
})(jQuery);
</script>
<?php
}
Code goes in function.php file of your active child theme (or active theme).
Tested and works.