On the qbo help page it says that estimates can be accepted or declined in the email, however the only way to get this is to go on web, copy link post in email then accep

Hi completecanvas-o,

Welcome to Community!  QuickBooks Online enables you to send estimates to your customers, so they can approve your work.  I can see how you and your customers would benefit from being able to accept or decline an estimate from the email they receive.  However,  this option isn't available at this time.  If you would like to see this option in future updates, please feel free to submit a suggestion to our Development team, by going to your Settings ⚙, then select Feedback. Enter your product suggestions, then hit Next to submit.

As a workaround, you can enter a Message displayed on estimate, or a Message displayed on statement to ask customers to respond to the estimate email to give their approval.

Once they accept or decline the estimate you'll manually update the status in your account, then convert the estimate to an invoice.  Here's a helpful article to guide you through the process of creating and sending estimates in QuickBooks Online.  

If you would like to see the option for your customers to click on a link to accept the estimates, please feel free to submit a suggestion to our Development team, by going to your Settings ⚙, then select Feedback. Enter your product suggestions, then select Next to submit.

If you have any other questions, feel free to reach out again.  We would be glad to help!