It wasn't the code that was the issue (and you probably wouldn't want an S3 compatible API). It's that on top of the Durable Store pricing, you need to pay for the $0.15/million requests + $12.50/million GB-sec processing + $0.09/GB data transfer.

Given the point of R2, the fact there's $0.045/GB ingress + egress data transfer (which has been recently reduced from $0.09) when using Durable Objects.

Given the original comment is basically about egress being too expensive therefore R2 is offering to for free, Durable Objects' Store isn't a particularly good alternative to just using S3 or DynamoDB unless you're already using Workers/Durable Objects.