The letter although I cannot confirm it is 100% legit it does seems that combined with the other leaks today that their is an element of truth to its contents even if the letter itself may be a fake. The pro-order bonus for Future Shop customers is an exclusive 'photo viewer' featuring screens which will not see general release. The letter begins by stating that Rockstar have 'just released' the second trailer for the game. As the pre-orders are due to begin from 5th November this would suggest that the trailer should be released in the next few days.
Also leaked today was a new piece of artwork which seems to featured on the posters which will be handed to pre-ordering customers.