You should be able to go to the cave, pick up the letter there and deliver it to the merchant still. I'm not 100% sure though, I'm afraid it's rather easy to mess this quest up if you don't do things according to a certain order (like the one on Kiya's website).

I am in blood
Stepp'd in so far, that, should I wade no more,
Returning were as tedious as go o'er.