Just when you thought there were no more spaces to put ads, the
keep ad™ appears in the UK, sandwiched between your sandwiches the keep ad can carry coupons, special offers, whatnot, and also doubles as a holder when you fold it to keep that other half of your sandwiches intact if you only eat one half at a time.

It just might be the perfect place to advertise origami classes and the Atkins diet. Or why not the "how to make paper airplanes" book? ;)