well I also have a pen pal in south korea, and I always send her a little gift but after a while I don't know what to send her anymore, but I always try to find typpical Dutch stuff like candies (Whilelmina peppermint, stroopwafels, drop, old dutch candy) but also stuff you could probably get somewhere else to but in a Dutch brand, such a chocolate, .. and others if I go somewhere like Amsterdam, or to the beach, I buy her those souvenirs, like little clogs.. and all very small, so I send them easy by mail and postage costs are nothing more then 2 euro, unless it's her birthday and I send something bigger. well I hope I help you with this
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ohw I forgot, if she really likes Harry Potter, you can send Harry Potter stuff, it doesn't matter that you can also buy it in Korea, it's always nice to get something that you really like