I am really not sure why such a basic feature is missing, but I would like the ability to get a specific purchase by ID.

According to the Simplero API documentation, you can look up a purchase by email address https://github.com/Simplero/Simplero-API?tab=readme-ov-file#find-purchase-by-email but if the person has purchased the same item more than once, it returns all of the purchases.

I have one business client where the manager uses the same email address to purchase training for all her employees, so that email address is connected to dozens of purchases for the same product. Really not helpful.

I'm hoping that this feature already exists and just isn't documented? In which case I'm happy to be wrong. But if it's not a feature, it's a pretty basic one that needs to be implemented.