Previously they only offered tiered billing (e.g.: gold, silver, bronze plans), further, the API for that was structured so you're billing for the next month.

Previously they only offered tiered billing (e.g.: gold, silver, bronze plans), further, the API for that was structured so you're billing for the next month. This announcement suggests "usage-based", which is how AWS and Google Cloud bill you: for how much you used during the previous billing period.