Hello there, willhuan.
Presently, we have an ongoing issue about unable to remove attachments on the transactions. That said, I'd recommend reaching out to our Phone Support team so we can add you as one of the affected users.
As workaround, you may untick the box next to the attachment to not include it when sending the transaction.
Also, if you want to customise the sales forms, you can check this guide: Customise invoices, estimates, and sales receipts in QuickBooks Online.
Let me know if you have additional concerns in managing invoices. I'm always here to help.
