Yeah, I second this. When I was at Square (a year ago), we knew the banks' APIs would allow "refunds" of arbitrary amounts, but it was a sort of "gee, isn't that odd" piece of information - it certainly didn't occur to me that it would be reliable enough to replace ACH. I'm super-curious about who made the call to use it this way.
The reason why it wasn't done to any great degree is that banks generally frown on this, and when I was last dealing with them a few years ago, Visa and MC would see a rise in refunds sort of like a chargeback. It wouldn't cost you a penalty, but do too many and you'd be at risk to get shut down.
