The developer (Stef) looked it up and said the discount was applied. The second payment cost more than the first, true, but that's a reflection of the number of cards added by it (that is, more than the Core sub).
But since there is still a small additional cost involved this way (fees, the per card costs are slighty different to get 'round' monthly prices) you now have an additional month of the sub.
But since there is still a small additional cost involved this way (fees, the per card costs are slighty different to get 'round' monthly prices) you now have an additional month of the sub.