Jabong.com — gst charges

Address:

Vaisakh house,kannikkal,kuttikkattukara p.o,eloor pathalam, Ernakulam, Kerala, 683501

I searched a product ojjasvi red self design maxi dress in having which costs 4049 rupees. And i applied one coupon awesome25 which gave me a reduction of 539 rupees and the final amount is 3510. But when i started shop the product it charged me additional 421 rupees as gst. How come a product costs 4000 rupees charges gst of 421 rupees???
Almost 10% as tax. How this is possible?
When i searched the same product in myntra i got no tax.