Flat rate is common in broadband access to the Internet in the USA and many other countries. A charge tariff is a class of linear rate, different from the flat rate, where the user is charged by the uploads and downloads (data transfers).
MUI stands for Multilingual User Interface and .dll.mui files are simply libraries(Dlls) that contain resources dedicated to localization : for an application to support multiple languages,dlls containing specific resources must be available eg language resources(arabic,french,english,chinese....
Error mapi.dll DLL errors are short for Dynamic Link Library errors and when these happen you know it. When a DLL error occurs on your computer you are generally notified via a pop-up style error message that will tell you what kind of DLL error you have.