Used stamps exchanges.

Postage 20 Rupees = 0.25€. I don't understand how such a cheap postage can be?

Dear friend, in fact Rs10/- postage stamps were affixed on the back side of the cover to make a total postage payable Rs30/- for the package weighing 0.20g.

Yes, in India postage is much less in comparison to other countries of the world due to currency variations and values of job.

OK, 30 rupees = 0.38 EURO
Within Slovakia, the postage is 0.50 Euro 2. Class is the lowest postage. So, 0.38 euros would not be enough to fill postage within Slovakia. So I do not understand how such a letter from India can be accepted in another country when it does not cover the costs even within the country. This is quite a strange postal policy.

See the cost of shipment for different countries here:
http://100stamps.club/discussions/messages/109