The Only Type of API Services I'll Use
What the OP fails to explain is how we can ensure that the user pays for the used service. The problem is that we have to bill after the service usage and the user could run away without paying and create a new account for his next use of the service.That’s the reason of upfront payment and the bucket usage payment model. An alternative would to pay upfront for a credit of usage, but this would require to monitor the usage  and an action when the credit falls below a threshold.The model to use depends on the type service we offer. If we can afford the loss due to people running away without paying the service, then it’s ok to bill after use. But that wouldn’t be a great business model.To me, the upfront payment of a usage credit seam the best compromise.