What a lovely letter!
I imagine now it would be 'As a token of our gratitude, please find enclosed a voucher for our Signature EasyJet Cheese Toasty®©™ [cannot be used in conjunction with any other voucher, offer or discount code]'
Isn't that little bit of metal you can see essentially like a little spring catch? Get something on the other side of it and I imagine that'll pop the front off.