Amazon vendor increases price after order citing GST

I ordered Your Amazon.in AK Multicolor Designer New Arrival Nett Lengha-Choli – For Woman, Girls
(http://www.amazon.in/dp/B0723CMG3G). order id (#406-5161006-1079540) on 30/06/17 at 399/- for cash on delivery but after 1/07/2017 they said in call as the rate increased to 699/- due to gst. It is not fair because when they placed my order then it’s only 399/- suddenly they changed the price after took of my order.

Pedada Prrasanna
Visakhapatnam 530007, Andhra Pradesh
Email: pxxxxxxxxxxx3@gmail.com