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.
A very specific folk.
Volksgemeinschaft is a German expression meaning "people's community", "folk community", "national community", or "racial community", depending on the translation of its component term Volk.
https://en.wikipedia.org/wiki/Volksgemeinschaft
> The concept was notoriously embraced by the newly founded Nazi Party in the 1920s, and eventually became strongly associated with Nazism after Adolf Hitler's rise to power.
(From your Wikipedia link.)