discount-based-on-payment-method-wc.php

<?php
add_action('woocommerce_checkout_init', 'payment_method_change_trigger_update_checkout_js');
function payment_method_change_trigger_update_checkout_js()
{
	wc_enqueue_js("$('form.checkout').on( 'change', 'input[name=payment_method]', function(){
        $(document.body).trigger('update_checkout');
    });");
}

add_filter('woocommerce_cart_calculate_fees', 'discount_based_on_payment_method', 10, 1);
function discount_based_on_payment_method($cart)
{
	if (is_admin() && !defined('DOING_AJAX'))
		return;

	if (!(is_checkout() && !is_wc_endpoint_url()))
		return;

	$targeted_payment_method = 'bacs'; // Here define the desired payment method

	if (WC()->session->get('chosen_payment_method') === $targeted_payment_method) {
		$discount = $cart->subtotal * 0.10; // 10% discount
		$cart->add_fee('Sconto bonifico', -$discount);
	}
}