Zaps are like a week old, there are only three clients that I know of that support zaps.

Amethyst, Snort and Damus.

The other thing is, the receiver has to be able to receive NIP-57 and not everyone has that set up properly.

The main thing that sucks about the current Amethyst implementation is that it is 1000 sats hard-coded (c.c. #[2])

Other than that it is by far the best Android client for nostr. It isn't even close. There is no second best.