FWIW (and for my situation, which is mostly smaller custom-designed ecommerce sites built on top of CMS's), I actually prefer this pricing model. 5% per sale is tough to swallow (this is a big reason I've avoided SnipCart). But basing it on site traffic (which is effectively what the per-API-call pricing is) is an easier sell to clients.

That being said, I agree that it's a little confusing to understand (or rather to guess how much one will be paying)... might be helpful to clarify if I need to guess ahead of time, or if per-month pricing automatically just happens based on the resulting number of API calls. (And if a rate limit could be set that would help too).

One more thought about this... what happens if my site gets DDOS'ed... what's the policy on avoiding (or not paying for) massive amounts of API calls that are accidental or bot-induced?