I think my question about how to use the inflight easyJet voucher is best posted here. I bought a £10 voucher at the reduced price of £8 during booking and added it to my booking. But I didn't know how to use the voucher as there was no separate link to it and I received no email about it. In the end I believe that it is encoded into your boarding pass and you just show this when purchasing something from the trolley.