Any chance you could convert pricing to $X per 1000 requests? My previous company needed something like this (we ended up building it our selves at a whooping cost) but our needs were 50k+/mo of calendar invites.
The existing tiers seem fine to me -- but I would change the name of Enterprise and make a top Enterprise tier which simply charges 1c/request, perhaps with a minimum of $50 or $100/month billed.