Gta registration

My client, Transporter has received notice from Service Tax Regarding amount of Freight collected during 2 previous years. However, in both the years his total takings have been below Rs. 10.00 Lacs.  This includes receipts in regards of Agri products, Transportation (taxable on reverse charge).

can you please state, what points to consider while answering the notice. you can also suggest me notifications to follow.