If you are charging labour charges for a product and in turn, manufacturer is paying excise duty on that product, then there is no liability of service tax. Pls go thorugh the following point in Mega notification no. 25-2012
30. Carrying out an intermediate production process as job work in relation to -
(a) agriculture, printing or textile processing;
(b) cut and polished diamonds and gemstones; or plain and studded jewellery of
gold and other precious metals, falling under Chapter 71 of the Central Excise Tariff
Act ,1985 (5 of 1986);
(c) any goods on which appropriate duty is payable by the principal manufacturer;
or
(d) processes of electroplating, zinc plating, anodizing, heat treatment, powder
coating, painting including spray painting or auto black, during the course of
manufacture of parts of cycles or sewing machines upto an aggregate value of
taxable service of the specified processes of one hundred and fifty lakh rupees in a
financial year subject to the condition that such aggregate value had not exceeded
one hundred and fifty lakh rupees during the preceding financial year;