[dxdm]

It sounds like a good way to make sure you don't overcharge your customers when handling such requests at scale. Failure and duplication will happen, and when serving enough requests will happen often enough to occupy engineering with investigation and resolution efforts forwarded from customer support.

Being prepared for these things to happen and having code in place to automatically prevent, recognize and resolve these errors will keep you, the customers and everyone in between sane and happy.

[hmaxdml]

These are all important concerns, but I'd go for an off the shelf library that does it for me (disclaimer I work at https://github.com/dbos-inc)

[vbezhenar]

You can just hire one person who will handle double charge issues and refund them when necessary. Might be much simpler and cheaper.

[dxdm]

Pressing a refund button is not why engineering gets involved. It's the cleanup of the related data, metadata and automatically generated documents, because these are not consistent anymore. Of course you can automate that, or make at least create more buttons for non-engineering people to push, but then we're back to spending effort to anticipate these problems and enabling the system to prevent and/or handle them.

You also need to think about what it means to double-charge your customers, what it means to them and their wallets, and to their relationship to you. Do you want their repeat business? What sums are we talking about? How do you find out about these double-charges, and how quickly? Do the customers have to complain to you first, or did you anticipate the problem and have things in place to flag these charges?

Yes, you can hire people in place of the code you didn't write, but that only makes sense if continuing to pay them is cheaper than writing the code once and then maintaining it, which also probably means the manual work generated should not scale in proportion with your business.

Finally, developing for more than the happy-path is not overengineering, it's plain old engineering. There is a point, a kind and size of business, where it makes sense to do these things properly, and then TFA comes into play. The cost of just winging it goes up and up, until you need to do something about it.