Dress Purchase Scam

I ordered a dress in Sasta Bazar. Initially they didn't mention any shipping amount for purchase. Later they are asking you need to pay shipping amount. After paying Shipping they asking about GST number. If we don't have GST number. They said we need to pay amount for GST. They are saying GST amount is refundable. But I purchase many thing in online but no one asking about GST number or need to pay for GST amount. It is new to me.They are forcing us to pay amount like shipping, GST amount etc. Otherwise our order will not dispatch. I said I will cancel my order but they said the order will cancel only at the time of delivery, there is no cancel of purchase before that. They said only if you pay the amount for shipping and GST, then only your order will dispatch.