I can see how the benefit of being able to add another field to your invoice email message would aid you in sending your transactions with QuickBooks Online (QBO), @LT23.

We take customer suggestions as opportunities to improve the various features within our products. Therefore, I would encourage you to send suggestions or product recommendations.

Your valuable feedback will be forwarded and reviewed by our Product Development team to help improve your experience while using the program. Here's how:

1. Go to the Gear icon.
2. Select Feedback.
3. Type in your recommendation in the Share your feedback field.
4. Click Next.
5. Follow the on-screen instructions to complete the process. 

I've attached screenshots below that show the first four steps.

Additionally, here's an article that'll further guide you in managing all your transactions and sales forms using QBO: QuickBooks Help Articles. You can click the + More topics button to view other selections. Then, utilize some video tutorials to learn your way around QuickBooks.

Please know that you're always welcome here in the Community if you have other reporting concerns and questions about managing your sales taxes in QBO. You can drop a comment below, and I'll gladly help. Take care, and I wish you continued success, @LT23.