can one business opt not to do business with another because a competitor requests exclusivity

My first question would be regarding contractual relationships. Does you company have a supplier contract with the smaller reseller? If so, the terms of that contract would determine whether you could legally quit selling to that company at this time. If there is no supplier contract in place, then you probably can give an exclusivity to the larger company.

I concur with my learned colleague. If you are not already contracted to supply flowers to the smaller competitor, you can stop doing business with them in deference to the wishes of your larger customer.

Exclusivity agreements are sometimes part of doing business. For example, if you buy an Iphone from Apple you have to sign up with AT&T for phone service. Your situation is not much different. You want to continue with the larger competitor, so you've honored their wish. That's legal, so far as I know.

This is not meant as legal advice as I don't hold Florida licensure. It's simply my analysis of the facts you lay out based on general principles of law, for educational purposes. If you need legal advice, please consult a lawyer who holds Florida licensure.

Good luck.