2) GST Law only required that certain mandatory fields which must be contained in the invoice.

3) The time period prescribed for issuing invoice is different for goods & services - for goods, it is anytime before its delivery and for services, it is within 30 days from the date of supply of services.

4) Small taxpayers, like small retailers, doing a large number of small transactions for up to a value of Rs. 200 per transaction to unregistered customers need not issue an invoice for every such truncation. They can issue one consolidated invoice at the end of each day for all transactions done during the day. However, they should issue the invoice where the customer so demands. (Note: Invoice of even Re. 1 to a Registered Person has to be issued compulsorily)

5) In normal circumstances, one copy of the invoice is required to be carried by the transporter. However, GSTN (Goods and Services Tax Network) provides a facility to obtain an invoice reference number and if a taxpayer has generated this number, his goods need not be accompanied by paper invoice during transportation. This mechanism helps to address frequently reported problems like paper invoices getting misplaced, mutilated, torn or lost in course of transportation of goods.

6) In order to keep compliance burden low for small taxpayers, GST law provides that taxpayers with an annual turnover of up to Rs. 1.5 crore need not mention the HSN (Harmonized System of Nomenclature) Code of the goods in the invoices.

7) Keeping in view a large number of transactions in the banking, insurance, and passenger transport sector, (such) taxpayers need not mention the address of the customer and the serial number in their invoices.

8) Where the goods are transported for delivery but quantity to be supplied is not known at the time of removal, the good may be removed on delivery challan and invoice may be issued after delivery.

9) No need to issue separate Bill of supply if VAT invoice is issued for non-taxable supplies. (I fail to understand what CBIC means by VAT invoice in GST Regime!!! Someone, please make me understand)

Source: CBIC