First of all, thanks for your fantastic feedback!
I hear your feedback regarding the subscription model, and we are going to make it possible to pay for the actual usage in the nearest feature.
In the meanwhile, there is nothing wrong to subscribe for 1 month only if you only need this occasionally:
P.S. I would really appreciate any suggestions to make the app and converter more convenient to end users. Feel free to reach me at alex (at) swiftify.com.