It could be that they have to collect sales tax on the total because it's a service. Perhaps, by the law of Florida, you the consumer aren't buying each individual grocery item, but rather you the consumer are paying a total price for a taxable service. I don't know this to be the case, but it sure sounds reasonable to me.
I can tell you this. I had a great conversation with Brian, the owner, last year. He impressed me as a very nice, very conscientious person. Briefly, I based it on this. I'd read here that my arrival day was sold out, that Garden Grocer wasn't accepting any more orders for that day. I either sent an e-mail to or left a message with GG to make sure this was true, to make sure it wasn't a malicious rumor. Brian personally called me back, and we talked for about 10-15 minutes. What it came down to was that the demand for his business was growing very quickly, and that he was in the process of hiring and training new employees. He said that he didn't want to accept my order because if he overworked his staff he risked them making mistakes and not providing excellent service to me and to the people who had already ordered and who had already placed their trust in him. Maybe it's just me, but when I offer someone money and he doesn't want it because he thinks he might not truly earn it from me, I'd trust that person with just about anything. If somehow Garden Grocer is inappropriately collecting some state sales tax, I'm sure Brian doesn't realize he is collecting it inappropriately, and would like to know about it so he doesn't continue.
But my bet is that whatever he is doing is 100% correct and legitimate.