Every person whose gross receipts or likely gross receipts of current financial year exceeds Rs. 9,00,000/-, such person is required to take registration as per Service Tax (Registration of specified class of persons). But such limit is not applicable in case where person are liable to pay service tax under reverse charge as in your case, then such person have to take registration within 30 days.
There are some conditions under goods transport agency regarding who pay service tax. like: tax on service of "Transport of goods by road", tax is payable by person who pay the frieght of service if consignor or consignee is company registered under company act, 1956.
It means if you are paying frieght you have to pay service tax, and you have to take registration with in 30 days from such date.
Others exemption
1) ST exempt when provided in relation to transport of fruits, vegetables, eggs or milk.
2) if services provided other than transport of fruits, vegetables, eggs or milk and any of following condition is satisfied:
a) when gross amount charged for an individual consignment is upto Rs. 750/-, or
b)When gross amount charged for all consignements carried in a goods transport carriage is upto Rs 1500/-.