bfwc_booking_cost.php

<?php
/**
 * Subtract last day's cost from daily booking
 * Requires bookings to be a minimum of 2 days
 */ 
function demo_dont_charge_last_day( $booking_cost, $product_id ) {
	$std_cost = bfwc_get_standard_cost( $product_id );
	$booking_cost = $booking_cost - $std_cost;
	return $booking_cost;
}
add_filter( 'bfwc_booking_cost', 'demo_dont_charge_last_day', 10, 2 );