Glad to see you here in the Community, @DaveJ.
I'll be sharing with you what I know about this issue of being unable to remove attachments in an invoice.
We receive reports about this issue. Our engineers are now investigating this matter. While we are working to resolve this issue, I recommend you contact our QuickBooks Online support team. They will add you to the list of the affected customers. This way, you'll receive notifications if there is an update about this issue.
Here's how:
- Sign in to your QuickBooks Online company.
- Go to Help (?).
- Select either tab to get started:
- Assistant: select a topic or enter a question. You can also choose the Talk to a human.
- You can also choose a way to connect with us:
- Start a chat with a support expert.
- Get a callback from the next available expert.
I'm also adding this article as your reference to customize an invoice: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Let me know if you have any other concerns about deleting attachments in a transaction. I'm just a post away.