Indeed, she could buy the flowers herself. We all could buy our own flowers ourselves. But what would life be if we used to do it?
Sometimes people forget their own loneliness as if it wasn't growing inside their hearts, their minds, their souls. If she bought the flowers herself, she would feel her loneliness like never before. It'd be very hard and the pain she would feel could certainly rape the only true thing that was in her: her feelings of love. And she would die, or at least, she would think about death (as she did).
And what about him? Would he buy the flowers himself?
Not at all. He had always prefered the silly instead of the true things. And now he is further from the truth than before, which means the flowers will probably not been bought. Nor himself, nor myself... None of us will buy that big and coloured bunch of flowers. But will the flowers always be waiting in that deep vase, full of water?