purchase

Place of supply for Bill to – Ship to transactions is defined under GST under Section 10 (b) of IGST;

(b) where the goods are delivered by the supplier to a recipient or any other person on the direction of a third person, whether acting as an agent or otherwise, before or during movement of goods, either by way of transfer of documents of title to the goods or otherwise, it shall be deemed that the said third person has received the goods and the place of supply of such goods shall be the principal place of business of such person

SO BY APPLIBG THE ABOVE IN YOUR CASE

POS WILL  BE IN TAMILNADU AND YOU HAVE TO CHARGE IGST