Learn Russian: Обычно мы не получаем груз от двух поставщиков. - Usually we do not receive cargo from two suppliers.

That rule only applies when the numbers 2, 3, and 4 are in the nominative (or inanimate accusative) case. When a number is forced into any other case—like the genitive двух here—that rule is thrown out. Instead, the noun simply matches the number's case, but in the plural. So both двух and поставщиков are in the genitive plural.

Could we put груз in the genitive case (груза) because the verb is negated by не?

Yes, you could, but it changes the nuance. With negated verbs, using the accusative (груз) implies a specific, concrete shipment that you expected but didn't get. Using the genitive (груза) would emphasize the total absence of the object, sounding more like "we don't receive any cargo at all."

Why use the imperfective verb получаем instead of its perfective counterpart?

The adverb Обычно (usually) tells us this is a repeated, habitual routine rather than a one-time completed result. Habitual actions always require imperfective verbs in Russian.