Fraud

I purchased some items from a seller and paid half the amount online in advance. After two days, I received a call regarding my parcel asking me to complete the full payment for "verification of bill" purposes. I was told that I could not cancel the order or receive my parcel unless I completed this additional payment. I have not been given my parcel nor a refund of the advance amount I already paid, and the seller is insisting on more payment before releasing the goods. I request a full refund of the amount I have already paid, or immediate delivery of the parcel without any extra payment, and request action against the seller for this fraudulent practice.