Reconciliation of Eway bills - Goods and Services Tax - GST

We are dealing in fabric from Gujarat and selling goods at Rajasthan and Haryana. While making sale invoice we generate E-way bill. Sometimes the customers return the goods without pre information to us. We come to know about sale return on receiving of goods on our door step. Thereafter we generate credit note for sale return without E-way bill. Customers do not generate Eway bills at the time of sales return. We have not been caught by GST officials while moving goods return. Now the GST department issued notice in DRC-01A for FY 2019-20 for difference of GSTR-1 and Eway bill. What to reply? What are the penalty provisions?