Hi there gabrielcarrieret,
Welcome to the QuickBooks Community! I appreciate how modifying the email messages sent with transactions to your clients is a great way to add your personal touch and make sure they have the information you need. Luckily, in QuickBooks Self-Employed it's pretty easy to do. I'll show you how.
When you create an invoice to send to a customer and then hit the Send button, QuickBooks will bring up a window where you can review the email addresses, the email subject, as well as the body of the email. On that page, you can make any necessary changes, including modifying the body of the email to say what you need. Once you've done that, simply hit Send and the message will be sent with the transaction.
If you're looking for something where you can save the message you're including there, that's not something that's available in QuickBooks Self-Employed at this time. It's a great idea though and I encourage you to submit your feedback about this by opening up the Assistant tool and typing in "feedback." Once you hit enter, follow the prompts to see how you can share with us the things you'd like to see in QuickBooks.
To have a feature like this more immediately, you can consider switching to QuickBooks Online, which provides more customization options for your forms, including the messages sent with them. Here are a few articles if you'd like to learn more.
I'll be here should you have further questions. Enjoy your week.
