It's nice to see you here in the Community, @JaneS72.

Customizing the appearance and layout of sales forms is a simple yet effective way to enhance your business communications. Currently, raising the header on the invoice template isn't available in QuickBooks Online. In the meantime, I recommend sending your suggestions and feedback about this feature to our Product Development Team. This way, they'll be able to add this feature for future product enhancements. Here's how: 

1. Click the Gear icon and choose Feedback.
2. Enter your feedback Suggestion.
3. Press Next.

We can track the progress of our feature request by visiting our Feedback forum page.

Moreover, I have a workaround on adjusting your invoice template to fit in your check envelopes, JaneS72. Please follow the steps below: 

1. Click the Gear icon.
2. Select Custom Form Styles.
3. On your invoice template, click Edit.
4. In the Content tab, select Table.
5. Click EDIT LABELS AND WIDTHS. 

         6. Toggle your Widths to adjust the Labels.

         7. Click Done to reflect the changes.

If you need to personalize and add specific info to your sales forms, please read this article for your reference: Customize invoices, quotes, and sales receipts in QuickBooks Online.

Keep me posted if you need further assistance with your invoices. Have a wonderful day!