These tests weren't checking anything structural about the APIs
beyond what's covered by the other tests. They represent edge
cases that we can check at a lower level instead.
It was also unclear what these tests were actually testing, as
the term "all cases" is vague. Looking at the test data, there
are variations in rates, multipliers and billable units for SMS
and letters, which I've summarised as "variable rates".
Note: I've removed part of the test data - for the first class
letter rate - as it's not clearly adding anything.