Okay, so I never bought coins in the Korean version, but I tried looking around. I don't know any Korean, so I had to use a translator.
https://gyazo.com/863bc7af7c79af5be50fcc8a612ddf55
The option that I had bubbled in the screenshot says "Overseas Credit Card." If that is selected, then the only credit card options they accept are JCB (Japanese), Discover, American Express, and Diners Club. So if you don't have any of those, then you cannot purchase them from the Korean version. I personally cannot because I only have Visa. Haha.
I have the physical book. It's the same ML. No time travel or anything from what I can see (I cannot read Japanese.) This is the same author as the one who illustrated Yowaki na Kimi to Nagai Shiawase.