Can I to add columns into invoices to denote purchase order and delivery note numbers for each item being invoiced?

Hi there, @kelly-palletsint.

In QuickBooks Online, we can customise sales forms to add specific info. However, we're unable to add columns for now. Though, it would be a great idea to have this option for customising invoices. I'll relay this message to our team in concern here on my end and add your vote for this one.

You can also share your thoughts and ideas through your account. Our product development team reviews all the feedback we receive to ensure we’re meeting the needs of our customers. 

Here's how:

1. Select the Gear icon at the top, then Feedback.
   
2. Enter your comments or product suggestions then select Next.
3. You'll be provided a list of suggested Help articles related to your comment. You can read through them or select Skip and send message.
4. From the drop-down, choose the appropriate category, then select Send message.

For your reference, check out this link: How to Submit Feedback. 

Let me know if you need more help with personalising your invoices. I'll be happy to help. Thanks again for reaching us and have a great day, @kelly-palletsint.