I would recommend a rating of “No Grade”. Mathematically, this does not affect the rating of the swap partner. In the comments, you could mention something along the lines of “lost in the mail”
If you're sure the package won't arrive and you're sure it was sent then give a “no rating” (not a 0). If the package does arrive you can always change the rating. I just finished a swap that took 30 days to get to me (~400 miles). Mine got there in 8 days.