It turns out that with the legacy Gmail Suite accounts you can no longer add extra storage space. This just came up because my sister’s account is running out of the default 15 GB space.
When that happened to me in 2017 I was able to add a 100 GB plan for $20/year. But my sister can no longer do that with her account. Since she also uses Apple Photos I’m going to ask her to clear out her Google Photos account. She can’t send or receive email at the present!
I myself only have 10 GB free space left.
Part of a chat I had today with Google Support: