nextTransaction has no ->totals; the figure is under ->details->totals.
Reading the property that does not exist evaluated to null and fell
straight through to '0', so every preview reported a next invoice of
nothing regardless of what was about to be billed. Caught against the
sandbox: Paddle said 4900, the mapping said 0.

Also carries creditToBalance, because "credited" on its own is the wrong
half of the answer. Paddle does not return the money to the card on a
downgrade — it puts it on the customer's balance and spends it on later
invoices — and somebody told only that they were credited will look for
it on their statement.

Split from 438ed6d136377961ebb7ffaa985743d11e55981b