It's a little worse than that. If you need to upgrade at least once per billing cycle, the system keeps charging you more and more (since an upgrade will always have a higher total cost than the previous upgrade) each cycle while accruing credits that will never start paying down unless/until you have a billing cycle without an upgrade. That's not a great design for most systems, but especially bad for a scalable image service intended to facilitate the flexibility of growth.

i think it's similar to how e-zpass works without the refund part

so they are overcharging and you need to rely on the refund so they can play tricks with their accounting