I don't understand how they can correlate a sale to a specific google account without getting personal identification data. By definition if they can associate a transaction with a user, the user has been identified and has lost all privacy.
Together with Location Services, they know everywhere you physically go, so given the last 4 digits of your cards and a transaction list it's pretty easy to guess who you are.
I don't actually know how to do this, so this is a bit like the "draw the rest of the owl" meme :)
If you assume there is an algorithm where Google can provide some "encrypted" form of their "users who have seen this ad" list such that the amount spent can be computed without knowing the intersection, you could exclude people who have opted out from the list Google inputs into the algorithm.