I'm not quite following your objection.. I'd expect a test that checks the multiplier is correct would detect orders of magnitude discrepancy. So if you're billing $x/byte you'd write a test for the billing thing that checks that, given y bytes, the bill is x*y.

[edit] This may need to be an integration test to be effective, there is a certain peril to mocking that could bite you here. But that's fine, we have the technology.

I think they're saying that their understanding of the hypothetical error is that somebody more or less configured a price as 5¢/byte instead of 5¢/GB. And if that's the case, that's not really a thing testing will catch. 5¢/byte is as "valid" a price as 5¢/GB.

There's a lot of hypotheticals in there, so who knows if testing could have caught this specific problem. But I see what they're saying in principle.