Dear Shri. Vivekanandan,
You are collecting interest for delayed payment of amount charged by you in your invoice under which the excisable goods are supplied to your customer. Such interest will not be treated as additional consideration under Section 4 of Central Excise Act, 1944. therefore you need not include such interest in the assessable value and pay central excise duty on such interest.
Collecting interest for delayed payment of invoice value by you customer cannot be treated as provision of service. If you collect any service charges for the same then you have to pay service tax.