Doc probably "sent" at least one earlier letter.
Knowing Doc, he probably wrote Marty a letter as soon as he found paper to write on and cash to pay for it. But his chosen method of delivery was... quite slow. As such, he could have gone to Western Union at any time and simply tell them "Hey, you know that one letter I gave you to deliver 70 years from now? Forget about it and take this one instead!" Doc was already paying for a letter to be delivered seventy years in the future, they were probably laughing so hard, swapping the letter for free seems like a common courtesy at that point.
From our point of view, we only ever see that one letter because it was the last one he gave Western Union. He could have written any number of letters prior to that final one, they just never got sent.
The letter specifically asks Marty NOT to retrieve him. If he really tried to fix the car, it means he had previously thought of going back, and would probably have sent a letter to that effect to Marty, just in case. After all, we can see in the movies that Doc is aware that the era he is stuck in is a very dangerous one (he has, for example, acquired guns and become proficient at using them).
This is pure speculation, but Doc seems to be intelligent enough to send a letter earlier than he did if he was thinking of leaving, since he could die for any reason, from banditry to dysentery, before being able to complete the repairs on the car. After he had resigned himself to staying, he put the car in the Delgado mine and changed the letter for the one telling Marty to go back to 1985 and destroy the DeLorean.