Hi there, WBioQ. Allow me to chime in and provide additional information about linking a transaction to your invoice.

Since this feature isn't available in QuickBooks Online (QBO), I suggest sending a request for feedback to our product developers about adding attachments. They may consider adding this feature to future updates.

You can follow the steps provided by my colleague above on how to send a feedback request.

In addition, you can refer to this article to learn how to record invoice payments in QBO: Record invoice payments in QuickBooks Online

Comment on the post if you still have concerns about managing your invoices in QBO. I'm always here to help.