No matter how much the price increases. If you bought when it was $5 , no matter future updates or price increase, you still can download the article, any files at any time. In the moment the purchase is done the article is linked to your itchio account.
If you lost the email, forgot the donwload links or similar go to the PixelFX page, click on buy and click at the bottom on says "I already paid for this":
Follow the instructions and add your email in which the purchase was done and you will receive an email from itchio with all the purchases associated to that email that should include "PixelFX Designer".