Both parties don't need to be online at the same time, you can email the whisper key to the recipient whenever you want, including while he's offline. The money will stay in the whisper address until the recipient uses the whisper key to sweep it into his wallet or send the money to someone else. There's no time limit, he can wait as long as he wants to get and use the whisper key.

Other than that you got it right. Another wrinkle with whisper addresses is that it involves a one-way communication and that is not automated at all, at least not in the current state. If the sender of the money refreshes the page or forgets to send the email or does any of the myriad things that people do to make a mistake, he will lose his donation and the intended recipient will never receive it. (That may be easy to mitigate by making the user click a button whenever they generate a whisper address. Then I could use an automatic email or something like nostr to automatically send the newly-generated whisper key to the recipient. But it would also make it easier to spam him with whisper keys that don't hold any value.)

Other issues that people pointed out on telegram include: