Set up custom invoice numbering

Add this code to site.php file:

Am_Di::getInstance()->hook->add(Am_Event::INVOICE_BEFORE_INSERT, function (Am_Event $event){
    $last_invoice_num = Am_Di::getInstance()->store->get('last_invoice_num');
    $invoice = $event->getInvoice();
    //public_id should be unique in database
    //in case you supply duplicate value here
    //aMember regenerate it with build in algorithm
    $invoice->public_id = date('Y_m_') . ++$last_invoice_num; //of course you can introduce your own algorithm here
    Am_Di::getInstance()->store->set('last_invoice_num', $last_invoice_num);
});

NOTE: DO NOT use dash ( - ) symbol in invoice numbers. There will be issues with payment processing because this symbol is filtered by most of payment plugins.