I can share additional information about QuickBooks Payments, @Service.

The third-party application that my colleague is referring to is another application that can integrate with QuickBooks Online (QBO). An app that can manage invoices, and it'll flow automatically to your QBO account. 

They can still pay you with a check. However, you'll need to record manually in QBO. 

Yes, you can still use QBO invoices without QuickBooks Payments. However, your invoices will no longer have the button to pay by credit, debit, Apple Pay, PayPal, Venmo, or ACH bank transfers. They must process it manually. Then, you may need to record the payments in the account. 

Refer to the following article for more information about cancelling your QuickBooks Payments subscription. 

If you no longer use QuickBooks Payments, here's how to record payments processed outside QuickBooks.

I'm always here if you need further information about cancelling your QuickBooks Payments account. Let me know by leaving a reply below. Keep safe, and have a wonderful day!