That's because their zfs account requires a dedicated IP and a VM.
It used to be 1TB minimum until pretty recently but they went with 4TB now.
If you want a less space solution, just spin up a t3.small (I suppose at least 2GB memory would be better for zfs) on AWS as Ubuntu with Cold HDD (minimum 125GB) as extra storage which is quite cheap at about 0.015/GB and install "zfsutils-linux" and you're good to go to use zfs.
