Learn Arabic (Modern Standard): استلم الطرد منا.

It is made of the preposition من (min, meaning from) and the attached pronoun نا (na, meaning us/our). Because من ends with an 'n' sound and نا starts with one, the two letters merge into a single, emphasized 'n' sound. It is written as منا (minnā) instead of keeping them as two separate pieces.

Could I say من نحن instead of منا for 'from us'?

No, you must use the attached pronoun نا (na) when a pronoun follows a preposition like من (min). Independent pronouns like نحن (nahnu, meaning we) are used as the subjects of a sentence, but they cannot act as the objects of prepositions.

Where is the word for 'he' in this sentence?

The word 'he' is built directly into the verb! The verb استلم (istalama) is in the basic past tense form, which automatically means 'he received'. In Arabic, you do not need to add a separate pronoun for 'he' unless you want to add special emphasis.

Why does الطرد (the parcel) sound like 'at-tard' instead of 'al-tard' when spoken?

The letter ﻁ (Taa) at the start of طرد (parcel) is a 'Sun Letter'. When the prefix for 'the' (ال) is attached to a word starting with a Sun Letter, the 'L' sound disappears and blends into the first letter of the noun. That is why it is pronounced 'aṭ-ṭard' and not 'al-ṭard'.