Your interface is rsync/scp/ssh.
They give you ZFS snapshots, you can use s3cmd from their machines, so you can delegate uploads to S3 via rsync.net.
Our prior backup setup was duplicity with GPG hitting S3, and this sometimes was flaky for listing the current keys.
Glad I read HN, I heard about rsync.net. They even have/had a HN discount. You should use the search functionality to find other threads.
https://www.rsync.net/products/platform.html
https://arstechnica.com/information-technology/2015/12/rsync...