The api returns letter details split by postage, so international
letters are returned with a postage of `europe` or `rest-of-world` not
`international` and these rows need to be added together when the rate
is the same before they are displayed on the usage page.
To do this, we need to replace the postage of `europe` and
`rest-of-world` with `international`. The data then needs to be sorted
by postage and rate before the letter units for rows which are
international and have the same rate are added together.
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