Learn Russian: Купи, пожалуйста, кефир и хлеб.

When asking someone to complete a single, specific action with a clear result (like buying groceries for today), Russian uses the perfective aspect. Using the imperfective покупай would sound like a general command to "always buy" these things or an impatient urging to "start buying."

Кефир and хлеб are the direct objects of the verb. Why didn't their endings change to show the accusative case?

Both кефир and хлеб are masculine inanimate nouns. For inanimate masculine nouns, the accusative case (used for direct objects) is identical to the nominative (dictionary) form. If they were masculine animate nouns (like animals or people), their endings would change.

Is it mandatory to put commas around пожалуйста?

Yes! In Russian punctuation, polite words like пожалуйста act as parenthetical words. Because it appears in the middle of the sentence here, it must be separated by commas on both sides.