One of my prior gigs was pushing a billion data points a day through DynamoDB without it breaking a sweat. We were paying for it, too--but it was there and it worked.
I stay by my comment that dynamodb is a joke wrapped in thick layer of marketing crap.
Edit: as other commenters have noted, you can perform a read after write on a new key.
Please think very carefully before architecting your app with S3 as a makeshift-database. S3 would be a valid option if you don't care about millisecond latency; don't require safe updates; never expect your application to scale past 100 requests per second; and don't have multiple query patterns for the same data (unless you're okay with several redundant copies of the same dataset). Consider just about any other database solution if this does not hold true.