Learn Arabic (Modern Standard): أرسل الرسالة والهدية معها.

It is a combination of two parts: the preposition مع (with) and the attached pronoun suffix ـها (her/it). In Arabic, when a pronoun is the object of a preposition, it does not stand alone as a separate word. Instead, it attaches directly to the end of the preposition.

Why does معها use the feminine pronoun ـها to mean "it"?

Because Arabic doesn't have a neutral "it" pronoun, you must use "him" or "her" based on the gender of the object you are talking about. Here, "it" refers back to "the letter" (الرسالة). Since الرسالة ends in the feminine marker taa marbuta (ة), it is a feminine noun, so it takes the feminine attached pronoun ـها.

How do we know أرسل means "He sent" since there is no separate word for "he"?

In Arabic past tense verbs, the subject is built directly into the word itself. The base dictionary form of a past tense verb, like أرسل, automatically means "he did" the action. If the subject were "she", "I", or "you", the verb would have a specific suffix added to the end of it.

Why is the word for "and" attached directly to "the gift" in والهدية?

The Arabic word for "and" is the single letter و (wa). A strict rule in written Arabic is that single-letter words cannot stand alone with spaces around them. Therefore, و must attach directly to the beginning of the word that follows it.