This is good manipulative psychology. The $18 is to anchor your expectation of the price and will appeal to people who just want to get the product as cheaply a

This is good manipulative psychology. The $18 is to anchor your expectation of the price and will appeal to people who just want to get the product as cheaply as possible. If you love the product, you can pay the "maximum", and because there's a range, it's feels like a ranking. The small increments mean that you don't feel too bad about the extra few dollars. As an improvement, I'd suggest giving labels to the amounts, like "I like CodeKit" to "CodeKit it awesome and makes my life so much easier!"