Learn Spanish (Latin America): Ella escribe las cartas y las envía.

No, they have different jobs! The first las in las cartas is the word "the" for a feminine plural noun. The second las in las envía is a direct object pronoun meaning "them" (referring back to the letters).

Why is the second las placed before the verb envía?

In Spanish, direct object pronouns (words like "it" or "them") must go right before the conjugated verb. So instead of saying "sends them", Spanish says "them sends" (las envía).

Why doesn't it say y ella las envía?

Once you establish who is doing the action, you don't need to repeat the subject pronoun. Since we already know Ella (she) is the one writing, it is perfectly natural to drop the pronoun for the second verb.

Why is there an accent mark on the "i" in envía?

The accent mark shows you where to put the voice stress when speaking. It breaks the usual pronunciation rules to make sure you emphasize the "i", making it sound like "en-VEE-a".