Quick fixes for high-level plight?

Pap dropping them leaves a huge amount of leeway for abuse. People afking in paps room without summoning on purpose to possibly take their kill/entrances so they get the cards/the other person doesn't.

True.

So I suppose it's back to putting them in cash shop with a level limit. There already are cash items with level limit, right? So Nexon don't need to invent a new mechanism, just put a number in there for 2x cards.
People could still buy them on a level 150 and transfer to a lower level, provided both are adventurers, but I figure not very many people are currently training adventurers on the same account as a high level adventurer, so the abuse potential is minimal.