On amethyst you can enter custom amounts & save them as buttons to click when you tap the zap button. That works fine & while I was going to say you could eliminate the extra tap it is worth noting that it probably prevents accidental zaps. Sending money is more costly than an accidental like. But if I have to type a custom amount I don't need to make it a button. So if the choice of fixed amount or a custom box appeared & you just either go with fixed or type & send then you could just have that show with a single tap or click.