Just wanted to circle back to this if anyone was curious since I had no success with the basics and the suggestions above. I consulted Blue Banyan to look into the issue and this is what they said:
Good morning, I hope you are having a good week so far. Our BB team has reviewed your template issue and you are correct in that this is unable to be changed at this time with NetSuite.
Since around 2021, NetSuite's APIs and customization options have not have provided a direct way to customize the intro text that accompanies the email carrying a PDF attachment. The default intro text you've provided seems to be a standard template used for sending transaction forms or documents via email from NetSuite. This is still not customizable and not resolved by NetSuite.
The workaround BB proposes is to create a new email template that will be used to manually send the email and choose the template (see image 1 and 2), or we can modify the workflow when users click on the "Email" button in the Customer Payment record.
We can create this workflow, just need the language to be used. Our estimated LOE 3-6 hours to build, test and deploy to Prod based on solution chosen.