Good point, Finwe! and a prime candidate as an example of
Success through Failure in Middle-Earth.
Once Aragorn began to trail the hobbits after they returned to the road, I think he would have taken any opportunity to introduce himself to them. So the fact that Barley waited a couple of hours to pass the letter on doesn't make too much difference. But definitely, the fact that he did not send the letter to the Shire has some connotations for the timing of events.
What would have happened if Frodo had received the letter on time? No chance meeting with Gildor, or perhaps more importantly Aragorn? Or would his early departure have meant he would reach Rivendell before the Nazgūl could catch up to him? I wonder if Merry and Pippin would still have been forewarned of Frodo's plan? They did almost as much for the fulfillment of the quest as Frodo and Sam. The
link I provided above is a good place to answer these questions.