@Lightwizard,
I agree with Lightwizard on the use of "if" instead of whether.
"Whether" to me implies a choice and I don't recall seeing it without normally listing choices: "whether to flee or fight" "whether to eat or not"
"Whether he can read it or not" is awkward. "If he can read it" or "if he is able to read it" are both acceptable but they appear to imply the postman has the letter in hand.
"Your handwriting on the envelope is so illegible, I wonder if the postman will be able to read it." is better if you are critiquing the envelope someone has just handed you.