Yes it mandatory to put HSN code or SAC in invoice for each product. Even tax payers paying under composition scheme need to show HSN code in bill of supply.
At the time of filing return invoice details has to be submitted along with HSN code, tax rate for each product and tax charged along with quantity and rate.
We at site are developing an invoice application which will be live next week, it will generate invoices as per GST invoice rules.
chatCommentsSAC or ACS should be understood as same. - Pulkit Sharma