Discount code raises already-insane shipping costs?

I fingered YN was reselling to other sellers. Probably under pressure from MH.
Oh, there's no pressure from M&H whatsoever. Make no mistake; YearOne holds the reins in that relationship. However, M&H is ridiculously busy with their similarly-excellent GM harnesses, on which they make all the money. The Mopar harnesses are icing on the cake, and they're always behind on those because it's not their primary business. It's a perfect situation for M&H, though. They profit nicely and never have to deal with a single customer-service issue. All their Mopar harnesses go to one address, and the folks at that address are responsible for the customer-side nonsense. How nice is that?

To clarify the M&H/YearOne harness situation completely:

M&H did not make any Mopar harnesses until they were approached by YearOne. YO owned the blueprints and the tooling at that time and still does. The wiring's been a YearOne exclusive since the first one was made. As @dadsbee said, M&H is handcuffed on this one. I've read the agreement; it's ironclad.

M&H has never sold a Mopar wiring harness to anyone but YearOne, period. Anyone offering M&H Mopar harnesses, now or in the past, bought it from YO. You might have gotten one elsewhere, but they got it from a warehouse in Georgia, guaranteed.