I signed up for a Melio account... and it didn't sync with QuickBooks Online... so Melio wasn't the solution either.  However, I found the solution to why QuickBooks wasn't accepting an ACH payment: the customer's bank was blocking transactions with QuickBooks. So, it turns out it wasn't a QuickBooks issue! Another thing I discovered... after calling QuickBooks (8 separate times), I could never get an answer... but, by using the CHAT function, I was able to communicate the issue, and within an hour they found the problem. Yea! Thanks everyone for your help... this was a tough one to uncover.