Btw: Small bug, you can only activate a Stage Upgrade when the amount of marbles you collected is larger than what you need. So in other words, you can only activate an upgrade that needs 12 when you have 13 or more, which is one too many.
Edit: This only seems to happen on Auto Drop when you activated the Auto Drop with the first upgrade. The first one is 5 and needs 5, but after that it also needs 5, but can only be activated when you have 6.