Change order totals based on payment method

index.php

add_filter( 'woocommerce_calculated_total', 'modify_cart_total_by_payment_method', 10, 2 );

function modify_cart_total_by_payment_method( $total, $cart ) {
	$chosen_payment_method = WC()->session->get( 'chosen_payment_method' );

	switch ( $chosen_payment_method ) {
		case 'bacs':
			$total += 2;
			break;

		case 'cod':
			$total += 3;
			break;

		case 'cheque':
			$total += 5;
			break;

		default:
			$modifier = 1;
			break;
	}

	return $total;
}

add_action( 'woocommerce_store_api_checkout_update_order_from_request', 'update_totals_on_payment_method_change' );

function update_totals_on_payment_method_change( $post_data ) {
	parse_str( $post_data, $post_data_array );

	if ( isset( $post_data_array['payment_method'] ) ) {
		WC()->session->set( 'chosen_payment_method', $post_data_array['payment_method'] );
		WC()->cart->calculate_totals();
	}
}