How do I attach a vendor invoice to a customer bill?

Hello there, cvaplumbing-gmai.  It's great to have you here in the Community. 

Let me assist you on how to attach an invoice to your customer bill. 

First, if you'd like to make your expenses billable to a customer, make sure to turn on the Billable Expense feature first. When done, follow these steps, 

1. Click the +New icon at the top, then choose Bill.
2. Enter the Supplier, Bill date, and Due date.
3. In the Account or Product/Service field, choose the expense account or product/service for the item or service you purchased for the customer.
4. Enter the Amount and the Description.
5. Choose a customer to be billed for the item or expense from the Customer drop-down.
6. Click the Billable checkbox.
7. Enter a Markup percentage, then if you need to charge tax for the item or service, click the Tax checkbox.
8. Click Save and close.

On the other hand, if you'd like to add attachments to your billing transactions, here's how:

1. Open the desired transaction.
2. Select Attachments.
3. Browse and select document you want to attach.
4. Select Open, then Save and close.

Lastly, I've also added this article in case you need assistance on how to record billable expenses in QBO: Enter billable expenses in QuickBooks Online.

Should you have any follow-up questions or concerns in managing your transactions, don't hesitate to post again here in the Community. Have a lovely day, and stay safe.