Service Tax Payment, how??

Dear all,

One of my client is running and Avertisement Agency. He do sales and also provide services like designing and marketing consultancy etc.

Now the problem is that, many times it happens that the bill may be raised for Sale as well as service. and the receipt received consolidatedly or by part payment. For eg. Bill raised for XYZ, Bill No. 1 Rs. 3500/- (Sales)  and Bill No. 2 Rs.7500/-. And receipt Rs.2500/-. How to define the liability of Service Tax in this case?

Thanks in advance.