It appears that Fastmail is introducing new plans and changing pricing. I have no idea how existing accounts will be affected, but in the past there were offers to extend current accounts at the old pricing.
You can see the new plans discussed in the Reddit thread by signing up for a free 30-day trial account. It appears that these conflict with what is advertised on the
current website. So I just sent an email to Fastmail staff I have met before, asking them about this pricing conflict for someone trying to sign up today.
Bill