I guess I'll have to give American eggs a try.

Since it was Lotte Mart's "Tongkeun Day," I took a look around... The price is a bit higher than the 3,390 won for a 30-pack at Costco, but since I could buy up to 2 trays, I got 2 trays of domestic eggs on sale for around 6,000 won, so buying 3 trays in total cost less than 15,000 won.
Since I have a Naver Plus Membership, I get free shipping from Lotte Mart if I spend over 15,000 won, so I added a few more items to get free shipping. What a steal...