@BetweentheFences 

Any chance this client uses journal entries for A/R or imports them via a JE?  If so, JEs can use multiple A/R accounts, and QBO will allow a user to receive payment on them just as it would for an invoice that posts to the default A/R account..

@BigRedConsulting 

"QuickBooks Online is generally weaker than QuickBooks Desktop. It has many limitations that Desktop does not have. Hundreds of them. Maybe thousands."

I think it's worth mentioning that QB Desktop is not really a competitor of QB Online.  QBO's competitors (Xero, Sage, MYOB, etc.), at the same price point, don't have multiple A/R functionality either so there's little incentive for Intuit programmers to add it.  Sure, salty Desktop users (myself included) aren't happy about losing the functionality of multiple A/R accounts, but Desktop has one foot in the grave and it ain't coming back.  Online has many advantages over Desktop too.