BSI stands for Business Software Incorporated.
BSI is a SAP partner that provide the US tax software engine that is used with payroll.
The BSI website is http://www.bsi.com.
Detailed contact information can be found in SAP note:
723946 BSI TaxFactory support
Latest information about TUBS delivered by SAP you can find at http://service.sap.com/tubs.
When you need to open a message to SAP refer to the component PY-US-BSI.