I don't think you can charge so little. You'd probably have to buy some amount of runs in bulk.

Another alternative would be you buy access for a block of time, but not an auto renewing subscription. Mullvad VPN works like this, I have to go into the app and re-up if I want to keep using it every month.

However I think this type of app should be a one time purchase anyway. Looks to me like it could work without any server / hosted infrastructure.