I'd expect the costs of running that HA stuff is an order of magnitude higher than the costs of even a few days of downtime p.a.. And migrating from Postgres to something with HA once needed is probably easier than migrating to Postgres if costs are killing you.
Yes AWS does it for you.
Redshift is not supposed to be used like Postgres. Redshift is a data warehousing solution, with completely different tradeoffs, and accommodating completely different work loads than your average Postgres database. For example, you can't create indices on Redshift tables or relationships between them, the consistency story is completely different from Postgres and Redshift is optimized for bulk loads, not millions of discrete inserts/updates a second.
Amazon RDS does provide hosted Postgres, and that is what you want to use if you want managed Postgres. Or you can use Herokus's hosted Postgres (it does get expensive with size, though.)
On top of that, Redshift is columnar database, which is an entirely different animal than vanilla Postgres (or any other DBMS).
Ref: http://docs.aws.amazon.com/redshift/latest/dg/c_unsupported-...