Stumbled across this yesterday on amazon. Ordered and received today, add two qualifying Nomad items and it will automatically remove £30.
Ordered two charge keys at £14.95ea total cost £0.00 free delivery with Prime.
Also works with the following items from same seller: Credit Card Charger
Credit to Overworked's post below, These qualify you for delivery of Addon Items. So you reach the £20 mark and only have to pay for your addon items.