I actually don't know what the cost would be for one chair and an umbrella. They normally do 2 chairs and an umbrella.
Probably $15 for one chair and an umbrella. Depending on the season, they do a week pass which is cheaper, but since you're going in March, may not be available then. No drink service at the chairs. You have the Perch bar there, for walk up service. Or, most people bring a small cooler with drinks and possibly some food, which it totally allowed on Club O beach. Much more economical.