Dear Mr. Kumar,
Yes. Your client liable for Service tax under Reverse charge mechanism as per Section 68(2) of Finance Act 1944.
Further, 75% abatement can be availed on gross amount & remaining only on 25% Service tax needs to be deposited.
Further, This service tax amount whatever you discharged this can be availed as input service credit if the same is for Inward transportation. For outward also it is eligible subject to your contract with your customers.
There will be no basic threshhold limit for making payment under Reverse charge mechanism.
Also refer 25/2012 Service tax sl no 21 Notification and 30/2012 Service tax.
Thanks