Vat/cst chargeable

My Question is:

Suppose i am a Registered Seller having business in Kolkata only, and received PO from Mumbai for supply of good but goods need to be dispatch to Westbengal. since the Invoice will be raise to Mumbai as they have advised, what should be charged to this kind of transaction 5% Vat or CST @ 2% agsinst C-Form or 5% without C- Form.

Please help .