@glb, in this type of sentence, you’ll find that on either side of the verb, you have a phrase that could act as the subject:
A melhor parte são os pequenos sabonetes e cremes! = Os pequenos sabonetes e cremes são a melhor parte!
With this type of construction, it is generally acceptable to agree either with the singular subject or plural object, according to Portuguese grammar standards (but not acceptable to have a plural subject followed by the singular verb). Some other examples:
O meu professor são os livros. = Os livros são o meu professor.
O resultado foram estes números. = Estes números foram o resultado.