If you’ve spoken to her calmly and explained how you see the money being spent and she still won’t listen you may have to do some behind the scenes trickery to make your wish come true.
You can explain to her that if she pays the florist in cash she could save some money and then take the cash to the florist, keeping some for other areas of the wedding.
If the florist is friendly and mom is with you maybe the florist will work with you on pretending the bill is more and giving you back a “refund.” Though I can hear the shock of me suggesting this as I type.
Remember that when money is involved there are usually some strings attached. The best route would be to talk to mom, get dad, grandma or a sibling involved in the conversation too to help bring her around.
If she just won’t budge then I’m sorry but you’ll have to find the money elsewhere.