An invoice can be forwarded directly via PEPPOL to your customer.
To do this, use the POST method with, for example /api/v1/invoices/1001?send_peppol as URL.
An example in PHP how to send an invoice to your accountant:
$email = 'luc@mymail.com';
$password = '123456';
$p = curl_init('https://app.simplybooks.be/api/v1/invoices/1001?send_peppol');
curl_setopt($p, CURLOPT_USERPWD, $email . ':' . $password);
curl_setopt($p, CURLOPT_POST, 1);
curl_setopt($p, CURLOPT_RETURNTRANSFER, TRUE);
$result_in_xml_string = curl_exec($p);