The climax would be when Lencho loses his crops to the hailstorm, which eventually leads to his writing of the letter to God as he needs the money.
The climax for A Letter for God is a humorous one. Lencho writes one last letter to God after receiving only seventy of the 100 pesos he requested. He asks God to send the missing pesos because his family is in such dire need of the money. Finally, he says that when God sends the rest of the money, he shouldn't send it through the post office, because the people working there are " a bunch of crooks." There is no falling action after this particular climax, which is a characteristic of climaxes that are essentially punchlines.
One might argue that the climax is when the hailstorm destroys Lencho's entire crop. The rest of the story from that point chronicles how this problem gets resolved, so it is certainly the main conflict. However, even the actions set in motion by this event lead up to that final letter, which is why it is more appropriate to call the last letter the climax of the story in this instance.