Our previous test ws returning a notification without a `sent_by`
attribute, which meant that cost was always 0.
Unfortunately, this meant that returning a real value for cost was
untested and (whaddya know) it broke immediately.
Old test scenario:
- billable_units=1, sent_by=None, cost=0
New scenarios
- billable_units=0, sent_by='mmg', cost=0
- billable_units=1, sent_by='mmg', cost=1
92897e20a8