Where in the world can I add a payment address for my invoices? Credit card payments are fine but my customers mail out a paper check.

You've come to the right place, @ireikhrud. 

Let's go to the Customer-facing address section to add your payment location details for your invoices. This way, your customers will use it when mailing out their paper checks. I'll guide you how. 

1. Go to the Settings (⚙️) icon and select Account and Settings under Your Company.
2. Select the Company tab.
3. Click Address. 
4. In the Customer-facing address section, either check the Same as company address box or manually enter the location info. 
5. Choose Save. 
6. Select Done. 

The screenshot below shows you the last five steps. 

Also, let's make sure you've checked the Street address box in the Content tab if you have a customized template, so this will appear on the invoices. See the screenshot below for your visual reference. 

Aside from the payment address, there are other things you can change on the Account and Settings page. To learn more about them, check out this article: Updating Business Info. 

When you receive the paper checks, I recommend recording the invoice payments from the + New button. It helps ensure your customer balances are up to date in QuickBooks Online (QBO). 

I'm here anytime you have other concerns. Enjoy the rest of your day. Take care, @ireikhrud.