public function get_headers() {
		return [
			'X-PAYPAL-SECURITY-USERID'      => $this->api_username,
			'X-PAYPAL-SECURITY-PASSWORD'    => $this->api_password,
			'X-PAYPAL-SECURITY-SIGNATURE'   => $this->api_signature,
			'X-PAYPAL-REQUEST-DATA-FORMAT'  => 'JSON',
			'X-PAYPAL-RESPONSE-DATA-FORMAT' => 'JSON',
			'X-PAYPAL-APPLICATION-ID'       => $this->application_id,
		];
	}//end get_headers