I don't understand what you think is a problem.

Heinlein is using poetic language to say, "There exist no free things delivered by other people."

This does include lunches, and the logic works: lunches, as a person-delivered thing, are not in the set of best things.

You might disagree with both premises (e.g. there are non-free things that are among the best things in life, like food when you're starving; and that people do in fact sometimes deliver things, like smiles, with no agenda).

But the logic of "the best things in life are free" doesn't suggest that free lunches are better than non-free lunches unless there exist lunches that are candidates for best-thing-in-life except for the problematic detail of not being free. You could go all ontological-argument-for-God at this and say: well, any conceptual lunch L that has a cost C would be even more appealing if C was lower, so the lunch L with C=0 is best; and therefore the lunch L with cost C > 0 is not best because there exists a thing strictly better than it. Or you could just say: you know, lunch is never that great. Some of the best ones have been necessarily pretty expensive, because good ingredients are expensive. I liked those a lot. But no lunch could ever count as a best thing. Or you could deny the premise: my lunch was expensive, and it was the best thing ever--stuff your stupid rule!