It could, theoretically, be like this on purpose, not only due to things like cargo space on specific planes, but also because this sort of pricing can have a psychological effect.
People don’t really know what shipping is supposed to cost, so the way they decide whether a price is good is by comparing it to other prices.
90 seems like a lot, but if it’s cheaper than the “economy” rate, people might be more inclined to purchase it and feel like they got a good deal.
Suspiciously similar to a trick parents use on their children. “Do you want to eat the peas first or the carrots first?” Gives them the illusion that they made a choice about what to eat.