Hi there, @daniel-lawrence.
Please allow me to step in and help share additional information on how to match payment in QuickBooks Online (QBO).
An unpaid invoice will post to the Debtors and Income or Sales account associated with your products and services. When recording a payment, the default account switched to the Bank where you've deposited the payment and will deduct it from your Debtors account.
If you've deposited the payment to the Undeposited Funds, the posting account will be your Undeposited Funds and the Debtors account. When you record a deposit, the Bank and the Undeposited Funds will now be affected.
Then, you can access the transaction journal and see the credits and debits for the said payment.
For more details about your transactions in QuickBooks check out this article: How accounts are affected by debits and credits
For detailed instructions on how to match a transaction, check out this article: Add and match downloaded banking transactions.
Let me know how this goes and leave a reply below if you need further assistance in managing your transactions in QuickBooks. I'm just a post away if you need help. Have a good one.