Kotka summer bus card is now on sale on Waltti website or bus ticket selling points around the region. The card can be bought during 1-31 of May and is valid 90 days from the first bus trip (however, no longer than 16.09.2017). Card can be bought for different travel zones Hamina—Kotka—Pyhtää. If you do not have a card yet it can be purchased at the price of 5 €. There are youth and adults price categories.
Buy summer bus card now and save: an adult travelling between Hamina-Kotka will normally pay 165 € for three months, with summer card only 100 €.
Additional information here (in Finnish).