aMember (site.php): Discount based on Qty

site.php

<?php

Am_Di::getInstance()->hook->add(Am_Event::INVOICE_GET_CALCULATORS, function(Am_Event_InvoiceGetCalculators $e){

    $e->insertBeforeTax(new class() extends Am_Invoice_Calc
    {
        public function calculate(Invoice $invoiceBill)
        {
            $discount_rules = [
                108 => [  //Product ID
                    1000 => 40, //Discount Levels
                    500 => 25,
                    200 => 20,
                ]
            ];

            foreach ($invoiceBill->getItems() as $item)
            {
                if ($item->item_type == 'product'
                    && isset($discount_rules[$item->item_id])
                ) {
                    $qty = $item->qty;
                    $discount = 0;
                    foreach ($discount_rules[$item->item_id] as $threshold => $percent) {
                        if ($qty > $threshold) {
                            $discount += ($qty - $threshold) * $item->first_price * $percent / 100;
                            $qty -= ($qty - $threshold);
                        }
                    }
                    if ($discount > 0) {
                        $item->first_discount = $discount;
                    }
                }
            }

            foreach ($invoiceBill->getItems() as $item)
            {
                $item->_calculateTotal();
            }
        }
    });

});