Sales invoice at 0 rate under gst

We are private limited co. in mumbai. We have prepared sales invoices at 0 rate to one of our customer as they does not have service tax number. We are not sure whether they have received GSTN or not. Previous year also we have prepared invoice to them without service tax. What points should I consider while making such transaction under GST regime. What type of documnets should I collect from party. transaction amount is 11000.