It's a one-time purchase, so once you've paid the initial cost of the software, you're set.
Affinity Photo is your best alternative to Photoshop. It's a one-time purchase, so once you've paid the initial cost of the software, you're set. In terms of power and performance, it's pretty much on a par with Photoshop, though it lacks some of Adobe's most advanced features like content-aware fill. (If you want an alternative to Adobe Illustrator, take a look at Infinity Designer.)