Slitherine Support is trying to get from Gamersgate the list of customers who bought PON. Baring that, they can't provide links based on screenshots or invoices, because there is potentially ten of thousands of customers from GG. At a time Paradox sold PON at a huge discount and even if most of the buyers never played the game a lot, there is a lot of people on Steam who possess PON.
So until a GG answer, operations are on hold on that, dixit Support.
Hofstadter's Law: "It always takes longer than you expect, even when you take into account Hofstadter's law."