Briefly, I wanted to discount sporadic bulk usage (hence expiry of 7 days) and charge fairly for long term availability (expiry of 2 years), with everyone else being in the 30 days bucket and treating it like a subscription.
I'm aware that a larger company would not want to deal with purchasing regularly and my thinking was that the 2 years option is great for that. Buy a bunch of credits for 2 years and forget about them. If you stop using the product, just don't buy again, I'm not going to nag you for money. I haven't set up alerts that your credits are expiring soon or you're running out yet, but that would be the next logical step if I stay with this model.
I'd love to hear your ideas for how else I could price Creodocs! I'm open to anything, this was just the result of trying to accommodate the most use cases.