For about three weeks customers could pay with both BTC and BCH even though Bitcoin Cash is about 15% cheaper. More painful for the company, however, was that if a customer requested a refund, Overstock refunded the original Bitcoin value — potentially causing massive losses. Unscrupulous shoppers could have exploited the bug to make huge gains in Bitcoin.
Overstock was the first major online retailer to accept the cryptocurrency Bitcoin in 2014. Currently, Bitcoin is trading at $13,958.80 and Bitcoin Cash is $2,575.08 according to coinmarketcap.
More: https://dowbit.com/overstock-bitcoin-cash-bitcoin-flaw/
They seem to have trouble with math.
If bitcoin was about $15000, a "15% cheaper" bitcoin cash would be about $12750. They probably meant it was about 15% of the price of bitcoin which is roughly where it has been.
And overstock understands the importance of bitcoin, they won't drop it.
(Or you might say, "Confused by the difference in math between 15% cheaper and 85% cheaper"? You're not alone. So is doubt.com.)