Hi there, @Costman99.

Thank you for posting in the Community. I'd be happy to help share some insights on how you can adjust the mailing address on checks to fit on the window envelopes.

You can edit the custom set up in your forms for window envelopes. Let me show you how: 

1. At the top right, click the Gear icon.
2. Select Custom Form Styles.
3. Fund the form and click Edit under the ACTION column.
4. Choose the Design tab and select Edit Print Settings.
5. Under Page margins put a check on the box next to Fit printed form with paystub in window envelope. 
6. Hit Done at the bottom of the page.

For additional reference, I've also included an article and screenshot for you: How to print forms for window envelopes.

If you need to reach out for more help, feel free to contact our Customer Care support for assistance. 

Here's how:

1. Sign in to your QuickBooks Online account.
2. At the top right, click the Help icon.
3. At the bottom, click Contact Us.

That should get you pointed in the right direction.

Drop me a comment below if you have any other questions about how to lower down the mailing address on checks. I'll be happy to help you out. Wishing you and your business success.