He bought a pizza yesterday is correct. He bought shampoo yesterday is correct. You would only use a determiner (indefinite article in this case) if you were to refer to 'a bottle of shampoo'.
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AnonymousI think when we put the indefinite article "a" in front of the word "shampoo," we talk of a type of it.Correct, and for this reason, we usually add more information to emphasize that we mean a type of shampoo.