I believe the current system is that - and MODs correct me if I'm wrong - if you max out on your cards, then that's that. You simply won't be getting a new card the next day and your subscription will renew on the same date originally specified.
This was done, not to stiff you, but it became a very complicated mess from Totem where users would buy a ton of credits and then sign up for a subscription and then buy all the cards with credits anyway. Complaints and the Customer Service went nuts. So, Totem has issued a statement in the past that stated that it is basically up to the consumer to logically decide if he or she will not overlap their on-going subscription purchases with credit purchases.
Hope I'm right and helpful. :-)
Mods double check this.