Thank you for your response.  I appreciate it, but unfortunately, neither of those apps appear to support access to an invoice URL.  As far as I can tell, the list of available fields does not include the invoice URL information.

I would like to be able to use the invoice URL on my customer portal so my clients can click the invoice URL and be taken to the same QBO payment page they would go to if they had clicked on a line in a "new invoice" email from QBO.  

My understanding is that the URL can be copied and pasted by hand from a QBO report, but I cannot find any way to access the URL using the QBO Salesforce Connector or any of the many third party apps I have tested so far.

I was hoping that someone know if this was currently possible without my having to set up an account with each third party app and spend the time testing each one.