And there's the magic of the Amazon marketplace. Let's you buy non-AWS services while still ending up on the AWS bill and contributing to any potential spending commitment the company has.

Does Amazon allow competing services to be sold on AWS marketplace ?

Specifically, if rsync.net service (for instance) were targeted as a non-Amazon backup location, would Amazon allow us to sell our service on AWS marketplace ?

This presumably competes with S3 and other storage options ...

I've never been on the seller side, so I don't know exactly what policies there are around what products can be sold. But to give an example, Datadog is available on the AWS marketplace, which offers a competing product to CloudWatch. I do believe you have to deliver your service via AWS somehow, such as via an AMI or container image etc but the service itself doesn't have to be on AWS as far as I know.

Doesn’t actually work because purchasing on marketplace is still restricted.

Indeed. I asked a dev to set up an ftp server. He was a decent Linux admin and had done this plenty of times on a £10 vps. This time he chose an AWS service that cost over a grand a month. It caused me lots of trouble when I got my credit card bill, and we were already well into the next month of charges. After that I had to take tighter control.

Same and it's going to be a huge, uphill slog to get the right people to consider localstack. But if we purchase a corporate license the savings would instantly pay for it so I feel compelled to try even though it is isn't really my day job to do so.

You can buy LocalStack on the AWS market place if you want to hide it in your infra bill ;-)

To be fair, that's only because nobody has figured out how to integrate "terraform apply" with SAP approval workflows.