The Postgres git repo doesn't make it easy to discover the employer of the commit authors, but yes, they do send patches to Postgres, here is a search of commits referencing mailing list discussions started by Amazon, plus a couple of examples of where they sent patches:
https://git.postgresql.org/gitweb/?p=postgresql.git&a=search... https://www.postgresql.org/message-id/92F458A2-6459-44B8-A7F... https://www.postgresql.org/message-id/9EF7EBE4-720D-4CF1-9D0...