Pro flowers and 1 800 flowers delivers flowers but they are not cheap and you do not get to see the flowers before you send them. Your best bet would be to purchase and drop off yourself.
There are many inexpensive flower companies that deliver in the USA. I am unsure if you are wanting to send flowers from the within USA or from another country to USA but ProFlowers and FTD both...
Proflowers has a wide variety of exotic green plants and flowers in their sister company called Proplants. These exotic species of plants and flowers are available fore delivery.