I understand how concerning this can be, especially when updating email addresses and pulling up different terms in different customers when it is not operating correctly, Joy. There are several reasons why this happens. I'll explain more and provide steps to help you navigate smoothly.
 

Firstly, when you set up a project with an email address, that email gets linked to all related invoices. If you update the email later, QuickBooks continues to use the first email you entered, not recognizing the change.
 

Secondly, the reason why Net 30 appears in the payment term for every invoice is because it is set as the default term. When you create an invoice, the system automatically applies this default setting. If you want to turn this off, I'll guide you through the process below.
 

Here's how:
 

1. Open your QuickBooks.
2. On the upper right corner, click the Gear icon.
3. Choose Account and Settings.
4. Select Sales.
5. In the Sales from content, click the Pencil icon.
6. Click the Preferred invoice terms dropdown then, make it clear.
7. Hit Save.
    

Once you've created an invoice, check out this article that guides you on how to send an invoice by email to inform your customer: Send an invoice to a customer in QuickBooks Online.
 

Moreover, you can refer to this article to learn how to set up and manage projects in QuickBooks: Create and manage projects in QuickBooks Online.
 

Let me know if want to learn how to automatically send reminders to customers or if you have any questions regarding invoices and QuickBooks terms, by replying to this thread. I'm here to help.