Welcome to the Community. It's great to have you connect with us here. QuickBooks is a powerful program that helps you get ahead with your work using the powerful features in the program.
The ability to customize a remittance slip in QuickBooks isn't available. I can see the advantage of having this feature and I highly recommend you send Feedback about this to our engineers. You can send feedback by clicking the Gear icon and looking for Feedback. New product ideas and improvements are largely based on the feedback we get from users like you.
Let me know if you have additional questions. I'm here to help in case you have any other questions.