When you click the "Download Now" button, it offers the free sample and suggests a $5 donation. I'm pretty sure that's what you paid, right? If you'd like to unlock all of the content, you need to manually change the $5 to $19.98, which is the minimum price to unlock everything.
I admit itchio's interface for this process is a little confusing, though. So if you want to "donate" the remaining balance ($14.98), I can go ahead and send you a download key that unlocks everything.