other strats about managing your lower tier cards:

1. make sure you only have the minimum (3) required per deck for tiers D/C/B (A too if ur only doing S)

2. max out the lowest tier cards in your decks in order (D>C>B>A) i.e. get level 3 on them, so they're gone from shop

3. if you need more slots (obv make sure you buy the upgrade for +2 slots) sell one of the lower tier abilities with the least impact to your dps -- the ideal is to have one slot for levelling+selling Piggy Bank

4. start locking one type of tier A/S card you'd like to level-3 first once they start appearing. make sure you still have enough slots for levelling/selling Piggy Banks (1 active slot + 3 inactive slots)

5. at some point you may have to sell more lower tier abilities to make space. it matters less once you hit shop 10, since all tiers will be 20% chance at that point

I slept on this till 2 cheevos left, wish i did it earlier lmao

...it also gives you tons of lifepoints! :)