I've never heard of tipping a flower delivery person, and I've worked in florist shops before, too.
I also work for a pizza chain, and you DO tip them.
But flowers? Generally NO.
Usually when flowers are delivered, they are delivered as a GIFT to the recipient from someone else. RARELY are flowers DELIVERED to the person who ORDERED them. As such, any "tip" is the onus of the PURCHASER, not the RECIPIENT. How would YOU like to get a gift and then have the person who GAVE you the gift expect a TIP for giving it to you? Same reasoning.
With Pizza delivery people, the people who are receiving the pizzas are generally the ones who ORDERED them, and thus, the tip is on the ORERERS.
Let's switch things around a bit.... A dear friend of yours calls the local Pizza place and says, "My friend, Mary is really SICK today, and I'd like to have a PIZZA delivered to her so she doesn't have to COOK tonight." So, she orders the pizza, gives the delivery instructions, and gives her credit card info to pay for it. Now... do you REALLY think that YOU (being "Mary") should tip the pizza delivery person when YOU didn't ORDER the pizza? It should be your dear friend who, on the phone giving the delivery/payment information that says, "Oh, and please add a $2 tip for the driver when you charge my card!"... RIGHT???
So... generally speaking, flower delivery people are not tipped for this reason.
Have a polite day.