Better to use filenames which are commonly used in the python word then
custom ones.

Previously, weight based shipping methods accepted an upper charge
field. When no weight band matched the baskets weight, this was
returned. I am not aware of a single retailer who calculates shipping
charges like that. It's impossible to pick a sensible value for a
retailer, as there's no upper limit to the amount of items ordered, and
any shipping charges would have to cover costs for that.

Not 100% sure if this is a good idea, but this commit adds a large
functional test that walks through most of the happy paths of the
shipping method dashboard. It's much simpler to write this journey in
one test method rather than separate test methods that require
progressively more set-up each time.