I'll take care of this query for you, @James1422.

In QuickBooks Online (QBO), email options are currently exclusive for invoices. The option to change an estimate to HTML when you email it is unavailable. A workaround I can provide is to download the PDF version of your estimate, then send it to your customer using an email outside QBO. 

Check out this article as well: Copy an estimate to a purchase order. You can use the steps from this article when you have an approved estimate from a customer, and you need to purchase those items from a vendor.

Feel free to drop a comment below if you have other questions about estimates in QBO. I'll be sure to get back to you. Enjoy the rest of the day!