I'm doubtful that it's a billion dollar industry. As a prior commenter noted, shippers will likely raise rates to compensate (particularly when you're a large shipper, rates tend to be negotiated anyway), or adjust policies to make refunds scarcer.

It may be a clever hack for a few early adopters, but is likely to be somewhat self-defeating if/as it grows. (Though in fairness, "shipment auditing" is a much bigger and more interesting play than refund harvesting specifically).