Sometime it can be difficult for uploader to provide the original paid reward source link if they get the images from other site like exhentai/kemono/.etc, so:

Do uploader need to provide the original fanbox/fantia/.etc source of free or non-free paid reward?
Or put the link where they get like exhentai/kemono/.etc instead?
Or add some tag like paid_reward for paid reward post?

For better? I think put original source (fanbox/fantia/patreon).
As for Exhentai, uploader put fanbox link in comment section (Example in: https://exhentai.org/g/1999716/6e683b7174/), so I can say thats redirect to fanbox artist.
I remember moderators say "just put Pixiv Fanbox in Source section" - post #728147 for example.