To be fair, it's not as clear as it could be that there are other "plus" plans. If you happen to land on the proton mail page when looking, they only show you the mail plus option (and unlimited). And even then really truncate what exactly you get for each paid option. There's a page that I was only able to find after opening my free account (it exists when not logged in, just never found it) that explains in depth all the options and differences.
Annoyingly, most of the individual upgrade pages don't give the 2 year purchase option either.