Levy of Service Tax on buying of customised software

Hello learned friends,

I am the legal manager of a software firm. As part of our IT solutions, , we buy/obtain license/sub license soft ware and customise/implement the same to our clients.

In a recent instance, the entity from whom we buy the software has charged Service Tax over and above the applicable CST. They might give online support in future if the need arises.

My question/dobut is whether they are entailed to charge Service Tax as per-se they have not done any customisation on the soft ware being bought from them?

Hope to recieve a positive reply with suitable/latest citations.