Still no idea why I am unable to pay a QuickBooks invoice.  I spent 1+ hrs. on the phone w/ support and that person said the issue was with the vendor's merchant services account settings.  Vendor spent 1.5+ hrs. on the phone w/ support and they said his setup was fine, problem must be on my end, pointing to incorrect routing number, insufficient funds, etc.  You know, all those boilerplate things that support put out there when they have no idea what is the real cause of the problem.

In the end, contacted the vendor by phone and he was able to process echecks for the 3 invoices, using the exact business checking information that I read off my "payment declined" screen and it worked fine for the echecks.  That proves that there was nothing wrong with my account information, same that I've been using for many years.

Support needs to have some sort of facility to either conference in both parties or be able to share issue information between the support folks.  The person I spoke with apparently saw an issue on the vendor side, but couldn't tell me any details.  The person the vendor spoke to indicated there might be a problem on my side, but again couldn't share any details with the vendor.  Such a waste of time on both sides for such a simple issue.  So glad my business doesn't rely on QuickBooks invoices for payments!