I appreciate the details you've shared to record the bill payment and credit card credit in QuickBooks Online (QBO), @dukexm.

You can create an expense to link the Unapplied Cash Bill Payment Expense and Vendor Credit. But you'll need to use the Accounts Payable account in the Category details section of the Credit Card Credit transaction to link it to the Vendor Credit instead of using an item or other accounts. Doing this can help you zero out the vendor balance.

Also, ensure picking the credit card account in the Payment Account field when you link the Credit Card Credit and Vendor Credit transactions.

Here's how to record an expense and link the transactions:

1. Click + New and then select Expense.
2. Select the vendor from the Payee field.
3. From the Add To Expense frame, click Add All to link the Credit Card Credit and Vendor Credit transactions.
   
4. Choose the Credit Card account from the Payment Account field.
5. Hit Save and close.

For more details about entering a vendor credit card refund and other refund scenarios, see this article: Enter vendor credits and refunds in QuickBooks Online.

You've mentioned that you purchased plumbing items for a specific project. You may use the Projects feature and create a bill to mark the expense as billable to the project. I'm adding this article to learn more about how your QuickBooks Desktop for Mac data converts in QuickBooks Online: Learn how features and data move from QuickBooks Desktop to QuickBooks Online.

Please don't hesitate to comment here if you need more assistance handling vendor and customer refund transactions. I'm willing to help you out more. Stay safe and well.