These socks have generally good reviews and, according to uk.camelcamelcamel.com have been sold for £14.99 since 06-Nov-2016, but were at £9.87 last October.
The lightning deal commences at 09:15, or 08:45 for Prime members. You can't buy more than one pair in a lightning deal, so would need to make up the order value to £20 with other items if free delivery is wanted. Many lightning deals sell only a small percentage, so crop up again over following days.
I have no connection with Amazon (other than as customer), the seller, or the manufacturer, and have not tried the socks, but thought they may be helpful to someone.