Amazon isn't particularly big in Korea, especially for delivery, but digital goods such as audibles is fine.. I'd ask for google cards here, or just resell them usually at 95~97% of face value, or full value to a friend. There are also local and regional chains with gift cards.

I still remember one time a relative gave me an LCBO (ontario liquor stores) gift card when I wasn't even in the country. That's completely useless. Fortunately his wife bought it off me

It also has intesting tax implications for buyer and reciever being a gift and all.

Not immedialtely obvious, but great for micropayments and the sharing economy. It's really good if you can get one for a contracter you hire for their favorite shop or restaurant as a personal touch/tip.