Discount Specific WooCommerce Payment Method (BTCPay in this example)

gistfile1.txt

add_action( 'woocommerce_cart_calculate_fees','ts_add_discount', 20, 1 );
function ts_add_discount( $cart_object ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;
    // Mention the payment method e.g. cod, bacs, cheque or paypal
    $payment_method = 'btcpaygf_default';
    // The percentage to apply
    $percent = 2; // 2%
    $cart_total = $cart_object->subtotal_ex_tax;
    
    $chosen_payment_method = WC()->session->get('chosen_payment_method');  //Get the selected payment method
    if( $payment_method == $chosen_payment_method ){
        
                   $label_text = __( "Bitcoin Discount" );
        
                   // Calculating percentage
                   $discount = number_format(($cart_total / 100) * $percent, 2);
        
                   // Adding the discount
                   $cart_object->add_fee( $label_text, -$discount, false );
    }
}
add_action( 'woocommerce_review_order_before_payment', 'ts_refresh_payment_method' );
function ts_refresh_payment_method(){
    // jQuery
    ?>
    <script type="text/javascript">
        (function($){
            $( 'form.checkout' ).on( 'change', 'input[name^="payment_method"]', function() {
                $('body').trigger('update_checkout');
            });
        })(jQuery);
    </script>
    <?php
}