Hi,
No, it is not possible with the iif import method. There is no way for QBI to know what the next Invoice number is, or if that number has been used "offline". This is a Quickbooks limitation. That's why QBI uses the PO number field. For Sales Orders, which is what you should be using if the customer has paid online, it will auto-number, but you still use the PO number field for the Zen Cart order number.
Thanks,
Adam
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