Trying to Spread the Message

These people say that a Muslim should follow only the Quran and not the ahadith of Prophet (S) and they gave the following example.

If a postman were to come to you and give you a letter, what would you do? Read the letter and follow it or start obeying the postman. Similarly, the Prophet Muhammad (S) was only a messenger (or a postman) and the Quran was the message (or the letter).

However, my question to them is if the letter asks the reader to follow the postman so that the reader can be lead to the sender, won't you follow the postman if you want to get to the sender. Similarly, the message/letter (Quran) says in Surah al-i-Imran, Chapter 3, Verse 132 and in many other places

And obey Allah and the Apostle.