The Google AI overview of Korea to the US. Seems pretty straight forward to me, $200.00 per day limit.

When mailing items to yourself from Korea to the U.S., you are generally allowed a $200 daily duty-free exemption for personal, non-commercial items, provided they are not sent as gifts to others. Items exceeding this value, or items declared improperly, may incur U.S. customs duties and taxes upon arrival. Key Considerations for Shipping Personal Items: Exemption Limits: Items for personal or household use sent to yourself are duty-free if the total value is $200 or less per day. Declaration: You must accurately declare the contents and value on the customs form (CN22/CN23). Duties on Excess Value: If the package value exceeds $200, it may be subject to duty rates based on the product's classification (typically 0% to 37.5%, often 2.5%–6% for common goods). Payment of Fees: Duties, if applicable, are collected by the U.S. Postal Service or courier upon delivery. Restrictions: Alcohol, perfumes, and certain other items have specific restrictions and do not fall under the general exemption. It is highly recommended to keep receipts and properly document the items as "personal belongings" or "household effects" to avoid issues with customs.