Clarification: it's free, but the only thing to pay for is a subscription, yearly or monthly. It used to be possible to purchase a lifetime subscription. Which, given what reddit did, I don't blame him for not offering that.
One time payment to remove ads will be available when the app is out of beta. Right now it has subscription option because people has been bugging the dev to let them send their money (including me, I've been waiting for this and instantly subscribe).