Hi there, Jun3. 

Thank you for reaching out in the Community. Yes, you can customize the email message and add their name from there. I'll show you how to do it: 

1. Go to the Gear icon.
2. Under YOUR COMPANY, select Accounts and Settings.
3. Choose Sales.
4. On the Messages section, click the Pencil icon. 
5. Below the Sales form menu, select Invoice.
6. Enter the following text as your email subject line: Invoice [Invoice No.] and/or your customer company name. 
7. Select Save.
8. Tap Done. 

You can check out the Customise invoices, estimates, and sales receipts in QuickBooks Online article for additional resources. It contains complete information on how to change the layout, design, and data fields that show on the invoices.  

If there's anything else that I can help you with, please let me know in the comments below. I'll be here to lend a hand. Stay safe!