Klarna sends us back a param used to populate `#gross_cents` and I use that here to populate `#gross` – I do it in this order since I can count on whatever Klarna sending us to be the most precise number available.

The `Notification` superclass appears to calculate `#gross_cents` from `#gross` (i.e. the other way around), but I think doing it the way I’ve done here is safer.