You don't have to pay for NPTO - it's free. That is, the four weeks I spent creating it on Linux (my daily OS) I'm giving away for free. It's work I like to do, so I don't see it as work, and thus I give it away for free.
But tediously porting each new version onto some other platform is work I don't enjoy, and neither would you. And because straight-up work isn't how I want to spend my free time, something has to compensate, and in this case, it's a unit price of $7.
As said, paying for NPTO is entirely optional: the program is provided for free for a free operating system. If you don't have Linux installed, you can get VirtualBox for Windows, and run an Ubuntu live image as if you were running a program on Windows. Saves you the $7, and is fine by me, too.
It's worth noting that I gave the same VirtualBox advice to a person who was asking for a Mac port of NPTO. (I don't own a mac, so can't develop for it.) His (predictable) response was that the whole VirtualBox thing would be too much work, and that NPTO isn't interesting or useful enough to bother. But the problem with his stance is obvious: if NPTO isn't useful enough for that person to spend a few moments installing VirtualBox, they don't have a good case to say that I should spend a far greater amount of time porting it over for them at no cost.
As far as the feedback goes, consider that $7 would hire a programmer for about 15 minutes, then consider how many multiples of $7 I'm likely to receive vs. how many weeks I spend working on the program. Then consider that most people who get NPTO will download the Linux version, which is free, and will get whatever benefit the program gives them. As such, feedback is arguably more of a community service than a gift to me.