You can use the client ID to fetch the billing contacts, e.g.
<?php
public function buildProcess(array $contact_info, ...)
{
Loader::loadModels($this, ['Contacts']);
if ($this->ifSet($contact_info['client_id'])) {
// Fetch all billing contacts for this client
$billing_contacts = $this->Contacts->getAll($contact_info['client_id'], 'billing');
}
}