When BetterTouchTool forced the switch to a "pay-as-you" want version (they even forcefully expired existing versions of the app, it simply stopped working unless you fork over money), their minimum was 4$ or so, because apparently if it was less, then the payment processing would be too high in relation.

So apparently in the bait-and-switch mentality of the mac world, that you can pay '0 dollars' is not obvious.