Is the $2.99 for Premium a one time charge or is that monthly? 
I believe it’s a one off thing. There is a patreon community too though for anyone who would like to make a regular contribution.
It’s a while since I did it though so apologies if I am wrong and things have changed!
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Yes, one time charge once you Buy into the App and like @siand said, you can also become a Patreon and donate monthly.
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To confirm what others have said, it’s a one-time purchase and it is then permanently unlocked for your Apple/Google account.
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