Actually, of course, this is an example of why that whole "two negatives make a positive" argument is fallaciously applied to language. If it were applicable, then this sentence - I ain't never had no gumbo taste like that before - would be completely acceptable.
Herb's problem, though, is he's socially inept. Intonation would tell him whether his customer meant by "I have not ever had any gumbo taste like this before" that he liked the taste or not. The number of negatives has nothing to do with it.