I think the game's final free update is coming up soon. From one of the latest news articles:
"REMINDER: This is the LAST FREE UPDATE to Zombie's Retreat! In October, the final release of Zombie's Retreat will launch for $6 here on itch and I am working on a Steam version, both with some new features & scenes. I will post an update about that build later this week to clarify more of the details about the update itself and the distribution process! ZR1 is almost fully completed!!"
But yeah, Steam Cards are probably a good buy if you can't pay any other way.