I think I'm primed from my days on Reddit where I found it just absolutely cringe when people would go out of their way to thank people for their worthless upvote karma, so on the one hand I lean towards not acknowledging. On the other hand, zaps are money; they have real value, so it feels a bit different. Ultimately, I'm with the people who say "expect nothing in return for your generosity." It's a pretty good stance for life generally. If you need some sort of pat on the back for your zap, just don't zap.