I really appreciate for your offer and I'm sorry for not getting back to you earlier!
I (currently) do not accept donations, as it would cause too much paperwork. I could add an option within the app but that would mean that (on the example of Germany) 19% would be subtracted for tax and another 30% would be subtracted for the store (e.g. Google Play). I don't really feel like that's a good way of accepting donations, right?
One thing you could do is - if you already have the permanent pro version - to purchase an additional subscription which will give me a little bit of money for the continued effort I put in the app.