Would GST be applicable on the portion of statutory levy charged by Automated Testing Station (ATS) for grant of fitness certificate to vehicles ? Also RCM applicable thereto. - Goods and Services Tax - GST

The ATS does not accept the full payment of the statutory levy as prescribed in the Motor Vehicle Rules. First the vehicle owner goes to the ATS, makes partial payment and gets an acknowledgement - then goes to the RTO which makes invoice on the full amount but the vehicle owner need pay only the remaining amount. After this the vehicle owner takes the receipt back to the ATS or fitness center, which then provides the fitness certificate. Lets say the statutory levy for heavy vehicle is Rs 1000 and the ATS takes part payment of Rs 400. Would GST be payable on this Rs 400 ? Also, would the vehicle owner be liable to pay RCM on the entire Rs 1000 ?