Learn German: Die Rücksendung ist umgehend eingetroffen. - The return shipment arrived immediately.

While both translate to "immediately," umgehend is more formal and very common in business, official, or customer service contexts (like dealing with a Rücksendung). You will often see it in written correspondence. Sofort is the standard word for everyday spoken German.

The current lesson is about the preposition binnen. Could I use it here instead of umgehend?

You cannot swap them directly. Binnen means "within" and must be followed by a specific period of time in the dative or genitive case. To apply the lesson's grammar here, you would have to change the phrasing to say it arrived "within a few days" (binnen weniger Tage).

Why does this sentence use ist instead of hat for the past tense?

The main verb here is eintreffen (to arrive), which describes a change of location or state. In German, intransitive verbs (verbs without a direct object) that indicate motion or a change of state always use sein (here conjugated as ist) as their helping verb in the perfect tense.

The past participle is eingetroffen. What is the infinitive of this verb?