Hi Tuan, I'd be happy to share the code if you'll agree to add the capability to send those emails for the backend

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I'm going to share it anyway, but I don't think that is the type of response you should send your customers (it's not even my request, but the request of the user internally who uses your software on a daily basis here on campus).
The reason why I think it would be best for you to add the feature in (especially if it's a feature she can simply toggle on/off via a checkbox when creating a manual registration...a checkbox such as "Send Notification Emails to User/Admin" or something similar...it can be off by default if there's a reason why you don't have it in the backend at the moment) is because if I do it there's a possibility we'd lose the feature down the road when we decide to upgrade again, plus it should be faster for you to add the feature than me since you're more familiar with your codebase and I would have to analyze more to figure out the proper way to add the feature in on my end.
Let me find the modified PayPal file so I can attach it here for you.