What has only two words but thousands of letters?

Question

There are thousands of antonyms, but not many pairs of opposite words differ by only one letter. See how many of them you can figure out.

Note: Although some pairs used here are not perfect opposites, they are listed as antonyms in the thesaurus.

1. Add one letter to "fast" to create a word with the opposite meaning.
2. Look around in this teaser to find two words that refer to opposite locations.
3. How many? Give two answers that differ by a single letter.
4. Start with a close companion; remove a letter and they become an evil person.
5. Complete with opposite meaning words: We are _____ ____ getting back together.
6. Add a letter to a private region to get something open to all.
7. Remove a letter from a private operation to get something visible to all.

Answer 1

Letter Box

"Letter Box" has only two words but thousands of letters posted by people to their loved ones.

Answer 2

Post office
its also has only two words but has thousands of letters.