To add additonal price on top of the calculated booking price

functions.php

<?php
/**
 * Add booking fee to price on product page.
 *
 * @param string $total_price Booking Price.
 */
function bkap_additional_booking_price( $total_price ) {

	// Here you can add additional condition according to business requirements.
	// This will add 15 to booking price for all the bookable product on the store.

	$new_price = $total_price + 15;

	return $new_price;
}
add_filter( 'bkap_modify_booking_price', 'bkap_additional_booking_price', 10, 1 );

/**
 * Add booking fee to price on product page.
 *
 * @param string $wp_send_json Array of data to be sent in wp_send_json function.
 * @param int $product_id Product ID.
 */
function bkap_show_additional_booking_price_info( $wp_send_json, $product_id ) {

	// Here you can add additional condition according to business requirements.
	// This will show Additional Price: $15 in the Price Box for all the bookable product on the store.

	$wp_send_json['bkap_no_of_days'] = $wp_send_json['bkap_no_of_days'] . __( 'Additional Fees: ', 'woocommerce-booking' ) . wc_price( 15 ) . '<br>';

	return $wp_send_json;
}
add_filter( 'bkap_final_price_json_data', 'bkap_show_additional_booking_price_info', 10, 2 );