Why is that an upsell? And what is so wrong with my response that 'shocked' you?
When you buy an app here, or at any place for that matter, you buy it for what it does and not for what you want it to do.
And if you want to be sure about something regarding the item that you are interested in purchasing, all you have to do is ask a pre sale question before you buy it. Like that, situations like this can be avoided.
That being said, if you need something extra added to the app, then it is more than normal that the app 's author should charge you for that. That is a common occurrence.