Just a WAG, and I know you are not doing any TRIMs in the field selection list, but try RPADding the parameterized fields:

SELECT Inv_type.inv_type_name, Invoice.grpid, Invoice.plnid,;
  Invoice.bill_date, Invoice.rx_amt, Invoice.claimfee_amt,;
  Invoice.rebate_amt, Invoice.comms_amt, Invoice.misc_amt, Invoice.amt_due,;
  Invoice.orgid, Invoice.inv_key, Inv_type.inv_type_key,;
  RPAD(Invoice.carrier_id, field_width) Carrier_ID, ;
  RPAD(Invoice.account_id, field_width) Account_ID, ;
  RPAD(Invoice.group_id, field_width), Group_ID ;
 FROM ;

Any way you can do a AFIELDS or COPY STRU TO EXTENDED before each requery and stick this info into some sort of log file so you can try tosee what fields are the offenders? You could also just trap this error and suspend to see if you can determine the peoblem.