Vat 5% to end user

Dear Sir,

I am from Chennai, Tamilnadu. I have purchased adhesive tapes from other state for 1.5% CST.

I have sold the same goods to a local buyer for 5%. They dont have the TIN number. They have only Service Tax Registered. 

The commodity code is 303 - 14.5% tax is applicable.

Since they are the end users, I charged 5% as VAT.

Now, I got a notice from Commercial Tax office to pay the remaining 9.5%.... 

Was I correct , or Should I pay the 9.5% out of my pocket?

Please clarify..

Thanks,