my_bfwc_flat_rate_with_overage.php

<?php
/**
 * Flat rate with daily overage for Bookings for WooCommerce
 *
 * When a product has the "Flat rate" option enabled, charges the flat rate
 * for bookings up to $min_days, then adds $daily_rate
 * for each additional day beyond that.
 *
 * Add to your theme's functions.php or a site-specific plugin.
 *
 * Note: designed for products set to "day" or "night" booking unit.
 * For weekly products, $$min_days would need to be expressed in weeks.
 */

/**
 * Append a per-day overage charge to flat rate bookings longer than the minimum.
 *
 * Hooks into the AJAX cost response so the correct price is shown on the product
 * page and submitted to the cart via the hidden calculated_booking_cost field.
 *
 * @param array $return_json   JSON sent back to the browser.
 * @param array $return_values Raw values from bfwc_calculate_booking_cost().
 * @param array $posted        Form POST data.
 * @return array
 */
function my_bfwc_flat_rate_with_overage( $return_json, $return_values, $posted ) {

	$daily_rate = 8;
  	$min_days = 14;
  
	$product_id = absint( $return_json['post_id'] );

	// Only applies to products with "Flat rate" enabled
	if ( ! function_exists( 'bfwc_is_flat_rate' ) || ! bfwc_is_flat_rate( $product_id ) ) {
		return $return_json;
	}

	// $return_values['num_units'] is the booking length in days (day unit) or nights (night unit)
	$num_units = (int) $return_values['num_units'];

	if ( $num_units <= $min_days ) {
		return $return_json;
	}

	$overage_days = $num_units - $min_days;
	$overage_cost = $overage_days * $daily_rate;

	// Apply the same tax treatment that bfwc_calculate_booking_cost() used for the base cost
	$product = wc_get_product( $product_id );
	if ( is_object( $product ) ) {
	  if ( 'incl' === get_option( 'woocommerce_tax_display_shop' ) ) {
		  $overage_cost = wc_get_price_including_tax( $product, array( 'price' => $overage_cost ) );
	  } else {
		  $overage_cost = wc_get_price_excluding_tax( $product, array( 'price' => $overage_cost ) );
	  }
 	}

	$return_json['cost'] = (float) $return_json['cost'] + (float) $overage_cost;

	return $return_json;
  
}
add_filter( 'bfwc_booking_cost_return_json', 'my_bfwc_flat_rate_with_overage', 10, 3 );