*/ function getOptions(); /** @param array $options */ function setOptions(array $options); /** @return string date Y-m-d start of subscription */ function calculateStartDate($paymentDate, Invoice $invoice); /** * Must this product be added to one of existing items * of the invoice, or this method may return NULL so item * will be added as new invoice line * @param array of InvoiceItem with same [item_type] * @return InvoiceItem|null */ function findItem(array $existingInvoiceItems); /** * Add quantity to invoice item - this method must return resulting value * @return int - resulting qty */ function addQty($requestedQty, $itemQty); }