If suppose you are unregistered dealer, and the customer to whom you are selling is a registered person. then the customer will pay GST under RCM, you will mention in the invoice. ₹100 price tax under RCM ₹18. customer will pay you ₹100 and pay tax ₹18 to govt. customer (receipient) will take credit of ₹18 paid. I may be wrong, please double check.