The "Hertz Member Program" one can be decent in some cases and terrible in others. On the whole, we find it one of the lowest performers on average.

To answer your question, "But is it normal for Hertz.com to be $168 more expensive? That's one heck of a deal through Delta."--it is normal, but it's not specific to Delta. It's that different CDPs provide different discounts on different rentals. There is no one "best" code. It depends on the situation. By not checking multiple discount codes, you are potentially leaving a very significant amount of money on the table.