Do I have to return the product to the seller? Who pays the shipping costs?

Unless the trader has offered to collect the goods himself, you should send back the goods or hand them over to the trader or to a person authorised by the trader to receive the goods, You should do it not later than 14 days from the day on which you has communicated your decision to withdraw from the contract to the trader.  If trader want you to bear the cost of returning goods after you change your mind, he has to clearly inform you about tha tbeforehand, otherwise trader has to pay for the return himself.