If I’m not mistaken, it’s a one-off donation, I don’t think you have to donate for every future version when you download it, but It’s a kind gesture to donate when future versions are released to help keep the development going.
Fritzing used a donation-based model to support its development, and the specifics of how it worked might have changed since then. However, I can provide you with a general understanding of how donation-based models typically work for open-source projects like Fritzing:
One-Time Donation: In some cases, a project may request a one-time donation each time you want to download MORE
I believe (but don’t know for sure) that the link in your donation receipt will allow downloads for about 1 year before a new donation is required (basically when your link no longer works you need to donate again. )