Thanks for informing us in advance that you’ve checked the delivery settings in your account, josh124. We understand that it's important to maintain professional email formatting since these communications directly reflect your brand's image. Let me help clarify some specific details regarding this matter.
Estimates and invoices are sent differently, particularly in how they are formatted in the email preview. Invoices will be displayed in a customized format, while estimates will appear as plain text.
Please note that the display in the Emails tab is merely a preview and not the actual format of the estimate.

To view the customized estimate, you will need to download the attachment. Just click the Download link or the PDF document as shown below.

We'd also like to let you know that we value all feedback and suggestions that can help us improve this feature. Therefore, we encourage you to submit a feature request to our development team, as this is a great way to share your insights on potential system enhancements.
Here's an article as a guide: Give feedback on QuickBooks products and services.
In the meantime, we want to ensure you're making the most of the current customisation options available. Let me share these articles for additional references about this:
We're always here to provide assistance when creating sales transactions, sending them out, and receiving customer payments. The QuickBooks Team is always available to answer them for you.