Sometimes there is little difference between "herself" and "by herself."
Generally "herself" means no one helped her.
Generally "by herself" means she was alone.
A child who makes breakfast for mommy might do it "herself" but maybe someone is there watching her.
A child who makes breakfast "by herself" MIGHT mean this, or might mean "completely alone."
In the case of the dress, I assume the person went to the store alone and picked out the dress without assistance. There is little difference in meaning.
Student or Learner