How do you call US numbers from Korea without paying expensive international call rates?

You do not need expensive international rates anymore. The simplest is a calling app that terminates to a normal phone line. For calling a US business like FedEx, which will not be on a chat app, use a VoIP service: Google Voice, if you have a US number, calls US numbers free over wifi or data, and Skype offers cheap per-minute rates or a subscription for US landlines and mobiles at a few cents a minute. Other low-cost options include apps like TextNow or Rebtel, and some let you call US toll-free 1-800 numbers for free. If you have a Korean carrier, you can check whether your plan has an international call add-on, but app-based VoIP over wifi is almost always cheaper. For FedEx specifically, also check whether they have a Korea local number or an online chat or KakaoTalk channel, which avoids calling the US entirely. A practical setup is to install Skype, add a little credit, and dial the US FedEx number directly over wifi for very little, with no Korean phone charges.