There doesnt seem a way to add Nectar to paypal. The paypal app does allow you to add loyalty cards but as of 25/8/17 Nectar isnt listed yet. I added Nectar to my Ebay account easily by going to 'My eBay' (top right) then on the left hand side click 'Nectar' under the 'Buy' heading. All Nectar approved purchases on ebay will then use your card automatically.
