Welcome to the Community! QuickBooks Online offers a flexible program that allows you to customize your receipts so they stand out to your customers. I'll be happy to show you how to edit the wording on your invoices.
You can add a custom message on your invoice by clicking on the Gear icon > Custom Form Styles > open the receipt template in question > navigate to the Content tab > from here you can edit the wording in each section of the receipt. I encourage you to add your terms and conditions here and make sure to save your changes once you're done.