The evidence from the story that shows the impact of the first-person narrator is the following:
"Bella never suspects that the letters are for Abuela because she doesn't know about Alejandro or that he used to call Abuela 'Bella.'"
This evidence aligns with the impact of the first-person narrator as it demonstrates how the limited perspective of the narrator, Bella, affects her understanding of the situation. Since Bella doesn't have knowledge about Alejandro or the history of his relationship with Abuela, she is unable to interpret the significance of the letters accurately. This limitation in Bella's understanding is a direct result of the first-person perspective, as the reader can only glean information from Bella's own experiences and knowledge.