How do I attach an invoice from QB Desktop

Thank you for posting here, manager37.

To move those attachments from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO), you'll need to make a copy of them. Then, reattach them manually to the transactions or your client's information in QBO.

The same place where your QuickBooks Company file is located also holds the "Attach" folder. You can turn to this article for more information about this procedure: QuickBooks Document Center: FAQs and common issues.

Then to add attachments to your QBO account:

1. Go to the Sales menu and select Customer. 
2. Locate and select the customer in the file to be attached. 
3. Click Edit at the upper right.
4. From the Customer information window, click the Attachments tab. 
5. Click the Attachments icon and open the file to add. 
6. Select Save. 

To add attachments to transactions:

1. Click the +New button from the left menu. 
2. Select Invoice as an example. 
3. At the bottom of the Invoice page, select the Attachments icon.  

If you’re adding multiple attachments, you can check out our QuickBooks App Store to utilize a third-party application that will enable you to add them most easily.

You can also look into this article for more details on how we handle attachments in QuickBooks Online.

Additionally, I got you this article that you can check to learn more about the limitation on exporting QBDT to QBO: What to expect when you switch from QuickBooks Desktop to QuickBooks Online.

Let me know if you have more concerns about QuickBooks Online. The Community always has your back.