Challenge for GST: How to process 3.5 billion invoices every month

India  is scrambling to finalize technology to process as many as 3.5 billion invoices each month as it moves to roll out a landmark nationwide sales tax from July 1.  The success of one of the world’s largest and most complex tax reforms depends on the glitch-free performance of the goods and services tax network (GSTN) -- the IT backbone of the new system that will eliminate a complex…