Any other stuff which include in mail multiplier times
Full explanation: After maintenance, updates or any other events developers send you messages with rewards. Messages you can find in your in-game inbox (left column in main menu). Once you get this message with DK reward you can buy this service. If reward include not only DK and also Diamond card (for example) you will also get it multiple times. It works if you DIDN'T take the reward from message. For example: Message have 100 Dragon Krystals and 1 random diamond card. If you buy 10.000 DK service you will get -> 10k DK + 100 random diamond cards.