Discount code raises already-insane shipping costs?

As noted above. Kevin King (YOne) owns the proprietary rights for a lot of M&H harnesses. Classics and Megaparts actually buy from YOne now to resell the M&H harness. M&H is handcuffed from selling direct.
That's a bit of a mis-statement. YearOne owns the rights. While Kevin King is the majority shareholder, he does not single-handedly own YearOne despite his desire for people to think so. There are a couple of entities who hold large pieces of it, one of whom alone has a 20% stake. Those shares were sold prior to/during Kevin buying as much of it as he could from Len Athanasiades when the latter wisely bailed. Kevin likes to take credit for a lot of things Len did, like founding the company, establishing its customer-service policy and record, and turning it into the giant it once was. His role prior to purchase was faithful "yes man" whose loyalty eventually promoted him from go-fer to sales manager, always under Len's thumb. Len was enough of a visionary to start, build, maintain, and bail at the right time.
What Mr. King is responsible for is wages frozen or reduced for nearly 20 years among long-time employees, drastically-reduced market share, extravagant management-team vacations, and foolish spending on projects that don't align with a restoration-parts company (the NOV8TR Camaro, Bandit "reissue", etc.). He's been blaming the economy for their downturn in fortunes since the 2008 economic crisis, despite everyone else recovering.

In fairness to KK, the ridiculous shipping thing is a leftover from the Len days. Len could not understand that warehouse employees are part of the cost of doing business, and Kevin (as always) agreed. So they timed the shipping process, averaged that and the wages of those doing the work, and calculated what percentage of the sale total should be added to the actual shipping. Len also didn't believe Summit/Jeg's were competitors, but then again they weren't selling quarter panels and Rallye wheels back when he had the tiller.