I'm not sure I quite understand the problem, but if I do understand it correctly, do this:
Make a switch that turns on when you deliver the letter, and then later, at the part where he says "Thanks for the letter", make a conditional branch, with the requirements of that switch being on, and if it is, he says "Thanks for the letter", and if it isn't, he says "Donkeys smell bad", or whatever you want.
Erm... I hope that made sense...