I've felt the same way thinking, I would buy $20 worth of credits but that's not a ball I'd use or restock when the 3 free disappear.
What I would do is buy $20 worth of credits if they added an additional amount of credits on top instead of balls. Let's say, the free Nike sleeve that they are giving cost 650 credits. If the offer was, buy $20 worth of credits and get 650 additional.
I think more people would be tempted.