Thank you for bringing this up in the Community, YWWP.
 
An estimate is a non-posting transaction that has a different purpose compared to an invoice. You can use an estimate when preparing a quote or a bid for your project proposal to your customers. By the time it will be approved by the customer, that’s the time you'll create an invoice.
 
This is the reason why QuickBooks Online feature for payment terms is only available in an invoice not in an estimate. Although, you can link both transactions as long as it's with the same customer.
 
To know more about estimates you can check this article: Set up and use estimates.

Let me know if you have questions in mind, I'm here to help you out. Take care.