Did the third level really exist ?

Did Sam intentionally place a letter in charley's bag of letters and why ?

Did Sam intentionally asked the coin store to say to charley that Sam had really brought 800 dollars of old currency so as to make charley free from his worries ?

1. No, the third level did not exist.

2. No, Sam did not place the letter in charley 's bag of letters. It was all Charley's imagination. He had cooked up stories about his visiting the third level and Sam following suit. 

3. Since the whole chapter is from Chaley's point of view, we do not get any idea about the true emotions and thinking of Sam. Thus, it cannot be interpreted whether Sam had intentionally asked the coin store owner to say to charley that Sam had really brought 800 dollars of old currency.