Just because she over ordered does not mean that the bride for that wedding paid for those Hydrangea.  You are wrong and off base to assume that she charged that bride for Hydrangea that were not in her wedding.  

Not sure why you would assume this or where you got the idea but you are being very cheeky. I would even go as far as saying you are questioning her integrity by implying she charged someone for something they never received.

  She is ordering, designing, picking up, processing and putting the flowers together.   In addition she spent time with you to come up with a plan. 
And then you also picked Hydrangea which is fairly expensive. 
If you think you are going to get her to lower the price by approaching her this way, not only will she say no,  I guarantee you will get less where she would have given more. 
If you want to get less then go ahead and insult her before she does your flowers. 
I am not saying she won't follow the contract and give you what she said because she will do so to a T.  You just won't get anything extra where she could have done or given more.
This is a lame way to negotiate and you send the message to her that you don't  respect her, what she does, or her time.  Maybe you could just tell her outright that you don't care if she has to take a loss, you want those flowers $5 cheaper and don't care who you have to walk over to get it. That may seem harsh but that is how you will come off.

A better way to approach this would be to ask her how you could get that arrangement that, "you fell in love with" and "so excited about" for $5 per table less.  Maybe you could switch from Sunflowers to yellow Daisy mums. Maybe you could go from 3 Sunflowers to 1 Sunflower per centerpiece and so on......
If you want to save money on delivery you could see if a friend can pick up the flowers and deliver them for you. 
Just talk to her nicely and comlimentary about this.  You have a problem( need cheaper $), it's her job to solve it. Be decent about it and let her do her job=
Make her like you! Make her want to help solve your problem and you will get more.