Hi
1. For inter state supply of goods, there's no threshold limit for registration. You need to get registered within 30 days of your first outward supply. However, since your case is export of goods, you need to take GST registration beforehand.
2. GSTIN shall have to be quoted on the shipping bill. So you need to take GST registration before first shipment only.
You will also have to apply for letter of Undertaking after GST registration for exporting goods without payment of IGST.
We may assist you with GST registration and filings.
Further you will need an Import Export Code for export of goods.
You can directly export through India Post by filing the requisite form.