I appreciate your effort in paying attention to the details given by my colleague, @dupolis. 

I'd also get frustrated if I also encountered this issue with the "attach to email" box in invoice attachments. I'm here to share some workarounds and information to help sort this out.

As provided by my colleague above, your customer will see the document or attachment as long as you’ve attach it successfully.  You’ll want to ensure that the files are attached first before you can see the “attach to email” box in your invoice. 

Please see this sample screenshot for reference:

You can also try deleting and recreating the invoice and adding the attachment again to see if the box will show up.

Otherwise, this could also be a browser-related issue. We can fix this by running some basic troubleshooting steps.

I suggest performing the steps through a private window. It's the best place to check browser-related issues while working with QuickBooks. To save you time, use either of these keyboard shortcuts:
 

• Google Chrome: Ctrl + Shift + N
• Safari 11 or newer: ⌘ + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P

 
If it works fine, you can go back to your original browser and clear its cache to start fresh. You can also use another supported and up-to-date browser to narrow down the results.

Moreover, please see this article for more details on the various file types that you can attach to any transaction in QuickBooks, as well as factors to keep in mind while emailing them: Attachments in QuickBooks Online.

You can always reply or reach out to us again if you need anything else about this issue about attachments in QBO. We're always here to guide and assist whenever you need help. Have a good one!