tl;dr:“AWS replicates S3 data between availability zones for you - whatever this might cost AWS is hidden away in the storage costs you pay for your data. So at its most fundamental level, this method is unlocking free cross-AZ costs - because you’ve effectively already paid for the cross-AZ cost when you uploaded your data to S3! Indeed, if you were to leave your data stored in S3, you’d end up paying significantly more than the cross-AZ cost - but by deleting it immediately after transferring it, you unlock the 99% savings we were going for.”