Customizing your invoice template is a great way to boost your brand image and personalize your experience using QuickBooks Online. I'd be happy to help you ensure that the next invoice you send is equipped with your changes.
The first step I'd recommend is to clear your browser's cache and cookies. Stored Internet files can slowly build up after time spent browsing the web, and clearing them out will help QuickBooks Online run at its best. Here's how to do it: How do I clear my browser cache and temporary Internet files?
The main step to pay attention to in order to make sure the invoice saves your customization properly, is to select the proper template at the bottom of the invoice creation screen by clicking Customize. If you've created alternative templates, this is where you'll go to select it. Otherwise, you can click "Edit Current" to customize the invoice you're currently working on.
Simple as that. Once you've selected the proper template, the invoice will send as a PDF with your customization saved. Give those steps a try and let me know if you have any other questions.
Let me help add some details about customising invoice template.
If you've already tried the steps provided by my colleague, @AddieC, the next thing you can do is to customise the standard template as well. There are instances wherein the actual design would flow from that template.
You may also create a new invoice template to check if it works. If the issue persists. I'd suggest reaching out to our Online Support to isolate this problem further. They have all the necessary tools to successfully apply the custom design on the actual emailed invoice.