Yeah, put another way: the "scale" is the $59 threshold. Cram as much as you can into an order, but keep it under $59 so the cheap shipping option remains available.
Cheap shipping is just that: cheap (ironically cheaper than comparable domestic shipments).
Past $59 and cheap shipping is no longer an option, and with that comes all kinds of extra fees that effectively doubles (or more) the total order cost.
Edit: I haven't tested this, but I'm 99.9% sure that 10 separate $59 orders (all with cheap shipping) will be cheaper - much cheaper - than a single $590 order where you have to use expensive shipping and nonsense fees. Just guessing here, but I'd expect the 10 separate orders to total around $700, while the single $590 order will likely be over $1200.
It's anti-economies of scale.