Import UPC codes into Square

A UPC-A barcode is twelve digits, and Square stores it in the GTIN field. That field takes 8, 12, 13 or 14 digits and nothing but the digits 0 to 9. The trouble starts in the spreadsheet, not in Square. Open a supplier CSV in Excel and a UPC that begins with a zero loses it. A valid twelve digit code then arrives as eleven, and Square refuses the row. Long codes turn into scientific notation for the same reason. Drop the file here and every code is counted digit by digit before Square sees it. Codes one leading zero short are grouped with a single fix. Codes that cannot be repaired are listed with their row number.