One Letter…

Answer: It's an envelope! You have to think about it literally; an "envelope" begins with E, ends with E, and only contains one letter–the "letter" is the letter you type/write. Some people believe it could be "eye" or "Eve" or "ewe", however, those things contain THREE letters (yes, you have to count the E's in each word because E is also a letter). Source: https://www.riddles.com/index.php/11385