It turns out they don't let you do that. "Free trial" means 30 days free, then it's Apps for Business or no Apps at all.
I felt (and, to some extent, still feel) extorted by that. The customer service experience was extremely dissatisfying. So, will I eventually use Google Domains? Probably. I just won't depend on it without a failsafe - which is what I should have done since the beginning.
Similarly, if you have Apps for Business like we do, you don't get to see the history of your support cases - that's in the Google Enterprise Support Centre, which becomes available when your user count reaches 100. They did enable it when I asked for it, but it's just very odd that you can have a paid service like this, yet not see your case history unless you're 'big enough'. I don't understand why the case history triggers on '100 users' rather than 'is paying us money'.
Overall GApps is nice... but there are a lot of rough edges and corner cases.