I think 15 % or 20 % Bonus packs hurts the economy of packs and cards. But with a small incentive of 5 % on buying at least 100 Packs I strongly believe that it will lead to more sales:
These 5 % only benefit those who hold and play the cards, because of market fees.
Only having a 5 % bonus won't lead to overprinting, because players have no reason of arbitrage. But it might cause players to buy a little bit more for themselfs.
This might be a win-win soulution.