Letter you received is not encrypted with anything complicated. No keys. No passwords. No computers required. Only one number.
Entire message uses classic ROT (Caesar) Cipher. Every letter has been shifted by exactly same amount. Your job is finding that amount.
Person who wrote letter had one number they liked for absolutely no reason. It appeared in jokes, random conversations... Maybe start there.
If you've known Shanks long enough... you probably already know which number that is.
If not... well... there are only 26 meaningful rotations to test. Or maybe... someone decided rules were meant to be bent.
Nothing hidden beyond this. No fake clues. No rabbit hole. Only one rotation separates nonsense from message.
Good luck.