I agree, but I could also see how someone would complain about that: “Our e-commerce site was taken down by Amazon right on our biggest day of the year. They should have just moved us up to the next tier.”
Seems like the most flexible option is to put a spending limit in place by default and make it obvious that it can affect availability of the service if the limit is reached.
My credit cards have credit limits, so it makes sense that a variable cost service should easily be able to support a spending limit too.
My credit cards have credit limits, so it makes sense that a variable cost service should easily be able to support a spending limit too.