Earlier quoted context omitted.
Come on, you spent $5 for an app. For a working app! This is a pretty low price. Actually, it's basically nothing. Of course you can't get support and are not entitled to feature requests at such a low price tag!
$5 is not cheap by app store standards. It's a fair price for an app in that class. As to whether you're entitled to support or feature requests, you can make an argument both ways. One could generate some word of mouth by following up on (simple) feature requests. In one of the apps, Fourier, the reviewers say that the horizontal frequency scale is off by a factor of 2 (but the frequency display elsewhere is accurat…
$5 is worth 3 minutes of developer time at common hourly contracting prices. If a developer receives and reads an email from a customer, they will probably end up with no profit. If they respond to it, let alone are adding new custom features, they are certainly losing money compared to working for someone else.