Hi there, 

I'm here to make sure you'll be able to customize invoices so that they fit the envelope window.

Let's make sure to set it up to fit the printed form in the window envelope. You'll find the instructions for doing so below:

To set up your forms for window envelopes:

1. Click the Gear icon, then Custom Form Styles.
2. Select the form you're using, then Edit.
3. Go to the Design tab.
4. Click Edit Print Settings.
5. Select the Fit printed form with paystub in the window envelope option.
6. Click Done.

Please check this article for more details on setting up forms for window envelopes and find out what forms and supplies are available for QuickBooks Online: How to print forms for window envelopes.

Please know that I'm only a few clicks away if you have any other concerns or issues. I'll be more than happy to answer them. Have a wonderful weekend!