I'm going to deliver flowers to a girl's house without her knowing
Do you have a reason why you would anonymously do this?
I want to make her happy
It's good to do such nice acts for people. This will make her really happy.
It's almost time, I'm excited!
What you're doing is good and harmless, but I do hope you find your courage to say that it was you so that they don't get the wrong impression that a stalker maybe the one sending the flowers. If you have intentions to date her, then you should come clean and introduce yourself sooner than later.