Hi,
So I understand that you don't want the invoice number to be generated for such orders.
The invoice number is only generated when the orders are confirmed.
And orders are not confirmed automatically if they are equal to 0.
So it should already do that.
But I suppose that you must have activated the "validate free orders" plugin of HikaShop to have them confirmed for HikariSerial to do its thing, right ?
In that case, why not use a new order status for the free orders (with the order confirmed status setting of the HikaShop configuration) so that HikaSerial could do its magic on both this status and the normal confirmed status, but have the invoice order statuses to not have that new order status ? That way, the free orders would still get the test license, but not the invoice number (and thus not the invoice).