Question

only the bearer knows where the shoe pinches elicidate by taking example from the letter

Answer 1

only the bearer knows where the shoe pinches .as in the story the letter everyone were making fun of old man Ali but only Ali was going through the dealing of love and separation no one else was feeling his pain .
the thing on which people were laughing on him( waiting for letter) was a hope for his love
thus the sentence clearly justifies

Answer 2

The line suggest that only the person who is in pain ,here patrol babu felts the pain grief and separation and had became a source of mean joke but when post master felt it he realises his mistake.