All international transactions (transactions done in foreign currency) done using an Indian debit or credit card are subject to conversion rates + an additional mark up fee usually in the range of 3.5% of the transaction amount.
The amount charged to your card is usually inclusive of the all the charges. There will be the 12.36% service tax on the 3.5%, so effectively it comes to about 3.9-4% of the exchange rate.
try paypal's conversion rate , if there is a option to pay via paypal and you have a paypal account . paypal charges only 2.5% and no service tax , the best part , you know the amount before it is being charged