Thank you for reaching out to us here on the Community page. I can share with you some information about sending estimates from QuickBooks Online.
At this time, there's an open investigation about emailed estimates that aren't formatted correctly. To give you more details, I recommend checking the answer provided by my colleague Jovychris_A from this link.
To learn more about estimates and how to add them to your invoices, you can as well check out the topics from this article: Set up and use estimates.
If there's anything else you need help with, let me know by adding a comment below. I'm more than happy to help. Have a great day!