I have kids in two schools that do the Spring flower sales.
Each is working with a local nursery. Basically we created Certificates for the amount of Money someone wants to purchase. The pto collects the money and pays the nursery the sales minus the profit percent agreed upon.
Then whoever purchased gets to take the certificates to the nursery and pick/choose and "purchase" what they want.
It takes a working relationship with a local nursery to make it work.
One school made about $40 dollars last year and is no longer doing it. The other about $1,500.
We're doing our first one right now. So far it's going well. We really wanted to try and find a way to have a fundraiser without the "guilt buying" so I think flowers really make sense. I'll pass along a couple of things to keep in mind. Most greenhouses don't do traditional fundraising, so you'll have to come up with your own order form. (not that difficult). Have a very organized way to file the order forms to make it easy for delivery. Try to have each person who orders come and pick up their own orders (instead of the seller). I don't know what part of the country your in but keep the weather in mind when you schedule delivery. (Will the flowers have to be kept inside or can they stay outside over night?) We're having ours delivered the weekend before Mother's day. Hope this helps, good luck