Yes, you're right, only a default template is sent using a mobile app. I have a workaround on what you can do about this, Akesto.

You'll want to use a web browser to select and use a template and send it to your customer. If you have already created the invoice template, follow the steps outlined below:

1. On the invoice page, click Customize and select the template.
   
2. Click Save and send.

On the other hand, consider sending feedback to let our product team know how this is helpful for your business. To do so, follow the steps given below:

1. Go to the Gear icon, then Feedback.
2. Enter your comments or product suggestions. Then select Next to submit feedback.

This goes to our Product Development team to help improve your experience in QuickBooks Online. You can also track feature requests through the QuickBooks Online Feature Requests website.

Moreover, I've added a comparison list that will guide you on the features that are only available on supported devices: Check out this article for more details: Compare Mobile App Features.

Please touch base with us here in the Community if you have any concerns about invoices or the mobile app. We're always right here to help you.