I’m also a Fastmail user (for the past 5 years or so), but I’m considering switching to iCloud, given the announcement it’ll start supporting custom domains. Fastmail is pricey ($50/month for the non-lite tier), whereas iCloud is $1/month for 50 GB.
Perhaps the user has confused fastmail with another service or they might have some legacy plan which is more expensive than it needs to be. (I think I have a legacy plan...)