Adding links for "paid reward available" posts

For images that also have a paid alternative version available, does it make sense to manually add a link to a specific Patreon/Fanbox post if the author hasn't included one?
For example: post #11651936 and Patreon: https://www.patreon.com/keiseeaaa/posts/raora-bra-strap-161892889

If so, what's the best way to format them to distinguish the original author's comment from the added information?

Adding it is fine if it seems useful to you, but it should be added as a comment. Commentary should generally not be modified for purposes like adding extra context, such extra info goes into the comments.