For reasons I won't go into - I'm using suitescript to loop through items on a SALES ORDER and force a specific tax code. For some orders this is resulting in a "TRANSACTION_IS_NOT_IN_BALANCE" or "TRANS_UNBALNCD" error. There are limited online resources for this error. Does anyone have any experience troubleshooting such an error?

From an accounting perspective, this sounds like you are modifying the transaction after it has posted. This will cause an imbalance in the amounts due to the change in tax.

I'm editing the record during after submit. So the tax codes are being updated just moments after record creation. These are not transactions in a closed period or anything. Is there anything I can do to bring the record back "into balance" or to delay the posting until after the potential tax code change?