Thanks for getting back to us, @PQH. 

Let me add details about changing the return and customer addresses suggested in the article about Print forms of window envelopes.

QuickBooks Online (QBO) allows six lines for printing your company name and address and another six for customer name and address. The company address depends on the information you've provided in the Accounts and settings. Meanwhile, the customer's address depends on the info you've set up on the customer's information page.

First, I suggest going through the steps recommended by my colleagues above about customizing the company and customer information through Custom Form Styles to fit them in the window envelope. During your editing process, please take note of the following templates and margin specifications: 

Next, if applicable and necessary, I also recommend condensing company and customer address information when details still don't fit the window envelope. Let me add that the information in address line 5 is only included if either Customer Name or Customer Company Name is omitted. Any text on address line 6 isn't included in the customer address.

When this is resolved, let me add Step 3 of this article about configuring your print settings as a future reference: Configure print settings.

You're always welcome in the QuickBooks Community should you have further updates about printing on a window envelope. Take care.